Decoding all that information on the sidewall

It's confusing isn't it? All numbers, letters, symbols, mysterious codes. Actually, most of that information is surplus to what you need to know. So here's the important stuff:

Encoded in the US DOT information (G on the diagram above) is a two-letter code that identifies where the tyre was manufactured in detail. In other words, what factory and in some cases, what city it was manufactured in. It's the first two letters after the 'DOT'. In my case, I've made a snafu but I can't be bothered to redo the graphic. My tyre example is clearly a Yokohama tyre, yet the two letters after the 'DOT' are 'A8' indicating Bridgestone tyre. Ho hum.
This two-letter identifieris worth knowing in case you see a tyre recall on the evening news where they tell you a certain factory is recalling tyres. Armed with the two-letter identifier list, you can figure out if you are affected. It's a nauseatingly long list, and I've not put it on this page. But if you click here it will popup a separate window with just those codes in it.

Tyre sizes and what they mean.

Okay, so you look at your car and discover that it is shod with a nice, but worn set of 185-65HR13's. Any tyre mechanic will tell you that he can replace them, and he will. You'll cough up and drive away safe in the knowledge that he's just put some more rubber on each corner of the car that has the same shamanic symbols on it as those he took off. So what does it all mean?

More recently, there has been a move (especially in Europe) to adjust tyre designations to conform to DIN (Deutsche Industrie Normal). This means a slight change in the way the information is presented to the following:

Metric Tyre sizes and the BMW blurb.

Fab! You've bought a BMW 525TD. Tyres look a bit shoddy so you go to replace them. What the.? TD230/55ZR390? What the hell does that mean? Well my friend, you've bought a car with metric tyres. Not that there's any real difference, but certain manufacturers experiment with different things. For a while, (mid 1990s) the 525TD came with arguably experimental 390x180 alloy wheels. These buggers required huge and non-conformal tyres. I'll break down that classification into chunks you can understand with your new-found knowledge:
TD - ignore that. 230 = cross section 230mm. 55 = 55% sidewall height. Z=very high speed rating. R390=390mm diameter wheels. These are the equivalent of about a 15.5' wheel. There's a nice standard size for you. And you, my friend, have bought in to the long-raging debate about those tyres. They are an odd size, 180x390. Very few manufacturers make them now and if you've been shopping around for them, you'll have had the odd heart-stopper at the high price. The advice from the BMWcar magazine forum is to change the wheels to standard sized 16' so there's more choice of tyres. 215-55R16 for example. The technical reason for the 390s apparently is that they should run flat in the event of a puncture but that started a whole debate on their forum and serious doubts were expressed. You've been warned

If you're European, you'll know that there's one country bound to throw a spanner in the works of just about anything. To assist BMW in the confusion of buyers everywhere, the French, or more specifically Michelin have decided to go one step further out of line with their Pax tyre system. See the section later on to do with run-flat tyres to find out how they've decided to mark their wheels and tyres.

Lies, Damn Lies and Speed ratings.

All tyres are rated with a speed letter. This indicates the maximum speed that the tyre can sustain for a ten minute endurance without coming to pieces and destroying itself, your car, the car next to you and anyone else within a suitable radius at the time.

'H' rated tyres are becoming the most commonplace and widely used tyres, replacing 'S' and 'T' ratings. Percentage-wise, the current split is something like this: S/T=67%, H=23%, V=8%. Certain performance cars come with 'V' or 'Z' rated tyres as standard. This is good because it matches the performance capability of the car, but bad because you need to remortgage your house to buy a new set of tyres.

UTQG Ratings

The UTQG - Uniform Tire Quality Grade - test is required of all dry-weather tyres ('snow' tyres are exempt) before they may be sold in the United States. This is a rather simple-minded test that produces three index numbers : Treadlife, Traction and Temperature.

The treadlife index measures the relative treadlife of the tire compared to a 'government reference'. An index of 100 is equivalent to an estimated treadlife of 30,000 miles of highway driving.

The traction test is a measure of wet braking performance of a new tyre. There is no minimum stopping distance, therefore a grade 'C' tyre can be very poor in the wet.

The temperature test is run at high speeds and high ambient temperatures until the tyre fails. To achive a minimum grade of 'C' the tyre must safely run at 85mph for 30 minutes, higher grades are indicative of surviving higher speeds (a rating of 'B' is, for some reason, roughly equivalent to a European 'S' rating, a rating of 'A' is equivalent to an 'H' rating.)

There are some exceptions: Yokohama A008's are temperature rated 'C' yet are sold as 'H' speed rated tyres. These UTQC tests should be used only as a rough guide for stopping. If you drive in the snow, seriously consider a pair of (if not four 'Snow Tyres' Like life, this tire test is entirely subjective.

Load indices.

The load index on a tyre is a numerical code associated with the maximum load the tyre can carry. These are generally valid for speed under 210km/h (130mph). Once you get above these speeds, the load-carrying capacity of tyres decreases and you're in highly technical territory the likes of which I'm not going into on this page.
The table below gives you most of the Load Index (LI) values you're likely to come across. For the sake of simplicity, if you know your car weighs 2 tons - 2000kg - then assume an even weight on each wheel. 4 wheels at 2000kg = 500kg per wheel. This is a load rating of 84. The engineer in you should add 10% or more for safety's sake. For this example, I'd probably add 20% for a weight capacity of 600kg - a load rating of 90. Generally speaking, the average car tyre is going to have a much higher load rating than you'd ever need. It's better to have something that will fail at speeds and stress levels you physically can't achieve, than have something that will fail if you nudge over 60mph with a six pack in the trunk.

Tyre constructions.

Simply put, if you bought a car in the last 20 years or so, you should be riding on radial tyres. If you're not, then it's a small miracle you're still alive to be reading this. Radial tyres wear much better and have a far greater rigidity for when cars are cornering and the tyres are deforming.

Aquachannel tyres.

In the last few years, there has been a gradually increasing trend for manufacturers to design and build so-called aquachannel tyres. Brand names you might recognise are Goodyear Aquatread and Continental Aquacontact. These differ noticeably from the normal type of tyre you would expect to see on a car in that the have a central groove running around the tread pattern. This, combined with the new tread patterns themselves lead the manufacturers to startling water-removal figures. According to Goodyear, their versions of these tyres can expel up to two gallons of water a second from under the tyre when travelling at motorway speeds. My personal experience of these tyres is that they work. Very well in fact - they grip like superglue in the wet. The downside is that they are generally made of a very soft compound rubber which leads to greatly reduced tyre life. You've got to weigh it up - if you spend most of the year driving around in the wet, then they're possibly worth the extra expense. If you drive around over 50% of the time in the dry, then you should think carefully about these tyres because it's a lot of money to spend for tyres which will need replacing every 10,000 miles in the dry.

TwinTire(tm) tyres.

This is an idea from the USA based on the twin tyres used in Western Australia on their police vehicles. It's long been the practice for closed-wheel racing cars, such as Nascar vehicles, to use two inner tubes inside each tyre, allowing for different pressures inside the same tyre. They also allow for proper run-flat puncture capability. Well, it seems that TwinTires have put the same principle into effect for those of us with roadgoing cars. Their system uses specially designed wheel rims to go with their own unique type of tyres. Each wheel rim is actually moulded as two half-width rims joined together. The TwinTires tyres then fit these double rims. Effectively, you're getting two independent tyres per wheel, each with their own inner tube or tubeless pressure. The most obvious advantage of this system is that it is an almost failsafe punctureproof tyre. As most punctures are caused by single objects entering the tyre at a single point, with this system, only one tyre will deflate, leaving the other untouched so that your vehicle is still controllable. TwinTires themselves actually claim a reduction in braking distance too. Typically from 150ft down to 120ft when braking from a fixed 70mph. The other advantage is that the system is effectively an evolution of the Aquatread type single tyres that can be bought over the counter. In the dry, you have more or less the same contact area as a normal tyre. In the wet, most of the water is channelled into the gap between the two tyres leaving (supposedly) a much more efficient wet contact patch. Time will tell whether this system is just a passing fad or if it will take off as a viable alternative to the standard wheel/tyre combos that we all use. Typical tyre sizes are 125/85-R16 and 125/90-R16 (Yokohama and Avon).
For an independent opinion on TwinTyre systems from someone who's been using them since the year dot, have a read of his email to me which has a lot of information in it.

Run-Flat Tyres.

Yikes! Tyres for the accident-prone. As it's name implies, it's a tyre designed to run when flat. ie. when you've driven over a cunningly placed plank full of nails, you can blow out the tyre and still drive for miles without needing to repair or re-inflate it. I should just put one thing straight here - this doesn't mean you can drive on forever with a deflated tyre. It means you won't careen out of control across the motorway and nail some innocent wildlife when you blowout a tyre. It's more of a safety thing - it's designed to allow you to continue driving to a point where you can safely get the tyre changed (or fixed). The way it works is to have a reinforced sidewall on the tyre. When a normal tyre deflates, the sidewalls squash outwards and are sliced off by the wheel rims, wrecking the whole show. With run-flat tyres, the reinforced sidewall maintains some height in the tyre allowing you to drive on. A pressure sensor is strapped to the inside of the wheelrim and is activated by centrifugal forces once the speed of the vehicle is above 5mph. It then samples the pressure once a minute for 4 minutes, and then the temperature once every 5 minutes. The information from all 4 wheels is relayed by radio to a dash-mounted readout for the driver's information. Of course, in normal use, this also means that the driver knows what all 4 tyre pressures are for everyday use. It means they're far less likely to get up one day and find one tyre with such low pressure that it's not possible to drive to a garage to re-inflate it. With run-flat tyres, that also becomes a bit of a moot point.

Both Goodyear (Run-flat Radials) and Michelin (Zero Pressure System) have introduced run-flat tyres to their ranges this year. The Michelin tyre technology cutaway explains it all much better than I can. Check it out here.

Not content with their Zero Pressure System, Michelin developed the PAX system too in late 2000 which is a variation on a theme. Rather than super-supportive sidewalls, the PAX system relies on a wheel-rim and tyre combination to provide a derivative run-flat capability. As well as the usual air-filled tyre, there is now a reinforced polymer support ring inside. This solid ring clips the air-filled tyre by it's bead to the wheel rim which is the first bonus - it prevents the air-filled tire from coming off the rim. The second bonus, of course, is that if you get a puncture, the air-filled tyre deflates, and the support ring takes the strain. Michelin say this system is good for over 100 miles at 80mph!

Remember up the top of this page where I was talking about tyre sizes and mentioned that Michelin had come up with a new 'standard' ? Imagine you're used to seeing tyre sizes written like this : 205/65 R15. If you've read my page this far, you ought to know what that means. But for the PAX system, that same tyres size now becomes : 205-650 R440 A. Decoding this, the 205 is the same as it always was - tyre width in mm. The 650 now means 650mm in overall diameter, rather than a sidewall height of 65% of 205mm. The 440 is the metric equivalent of a 15inch wheel rim - and metric is no bad thing - and finally the 'A' means 'This is a PAX system wheel or tyre'.

Wheel Information.

Okay. If you want to change the wheels on your car, you need to take some things into consideration.

Number of bolts or studs
It goes without saying that you can't fit a 4-bolt wheel onto a 5-bolt wheel hub. Sounds obvious, but people have been known to fork out for an expensive set of wheels only to find they've got the wrong number of mounting holes.

Pitch Circle Diameter
Right. So you know how many holes there are. Now you need to know the PCD, or Pitch Circle Diameter. This is the diameter of the invisible circle formed by scribing a circle that passes through the centrepoint of each mounting hole. If you've got the right number of holes, but they're the wrong spacing, again the wheel just won't fit.

Inset or outset
This is very important. Ignore this and you can end up with all manner of nasty problems. This is the distance in mm between the centreline of the wheel rim, and the line through the fixing face. You can have inset, outset or neither. This determines how the suspension and self-centring steering behave. The most obvious problem that will occur if you get it wrong is that the steering will either become so heavy that you can't turn the car, or so light that you need to spend all your time keeping the bugger in a straight line. More mundane problems through ignoring this measurment can range from wheels that foul parts of the bodywork or suspension, to high-speed judder in the steering because the suspension setup can't handle that particular type of wheel. This figure will be stamped on the wheel somewhere as an ET figure.

A real example They say a picture is equivalent to a thousand words, so study this one carefully. It's one of the wheels off my own car. Enlarged so you can read it is the wheel information described above. You'll notice it reads '6J x 14 H2 ET45'. The '6J x 14' part of that is the size of the wheel rim - in this case it has a depth of 6 inches and a diameter of 14 inches (see the section directly below here on wheel sizes for a more in-depth explanation). The 'J' symbolises the shape of the mounting surface of where the tyre sits. This runs around the front and back of the wheel rim. You may have noticed that some wheels have a 'JJ' description; this is mainly on 4x4s as the tyre can be held on more securely with a double mating face.
The 'H2' means that this wheel rim is designed to take 'H' speed-rated tyres. The 'ET45' figure below that though symbolises that these wheels have a positive offset of 45mm. In other words, they have an inset of 45mm. In my case, the info is all stamped on the outside face of the wheel which made it nice and easy to photograph and explain for you. On most aftermarket wheels, they don't want to pollute the lines and style of the outside of the wheel with stamped-on information - it's more likely to be found inside the rim, or one one of the inner mounting surfaces.

Matching your tyres to your wheels.

Okay. This is a biggie so take a break, get a hot cup of Java, relax and then when you think you're ready to handle the complexities of tyre matching, carry on. This diagram should help you to figure out what's going on.

Wheel sizes

Wheel sizes are expressed as WWWxDDD sizes. For example 7x14. A 7x14 wheel is has a rim width of 7 inches, and a rim diameter of 14 inches. The width is usually below the width of the tyre for a good match. So a 185mm tyre would usually be matched to a wheel which is 6 inches wide. (185mm is more like 7 inches, but that's across the entire tyre width, not the bead area where the tyre fits the rim.)

Rolling Radius

The important thing that you need to keep in consideration is rolling radius. This is so devastatingly important that I'll mention it in bold again:rolling radius!. This is the distance in mm from the centre of the wheel to the edge of the tread when it's unladen. If this changes because you've mismatched your new wheels and tyres, then your speedo will lose accuracy and the fuel consumption might go up. The latter reason is because the manufacturer built the engine/gearbox combo for a specific rolling radius. Mess with this and the whole thing could start to fall down around you.

Why would I want to change them anyway?

A good question. Styling and performance are the only two reasons. Most cars come with horrible narrow little tyres and 13 inch rims. More recently the manufacturers have come to their senses and started putting decent combinations on factory cars so that's not so much of a problem any more. The first reason is performance. Speed in corners more specifically. If you have larger rims, you get smaller sidewalls on the tyres. And if you have smaller sidewalls, the tyre deforms less under the immense sideways forces involved in cornering.

So how does it all figure out?

Point to note: 1 inch = 25.4mm. You need to know that because tyre/wheel manufacturers insist on mixing mm and inches in their ratings.
Also note that a certain amount of artistic licence is required when calculating these values. The tyre's rolling radius will change the instant you put load on it, and calculating values to fractions of a millimetre just isn't worth it - tyre tread wear will more than see off that sort of accuracy.  Lets take an average example: a car with factory fitted 6x14 wheels and 185/65 R14's on them.

Radius of wheel = 7 inches (half the diameter) = 177.8mm

Section height = 65% of 185mm = 120.25mm

So the rolling radius for this car to maintain is 177.8+120.25=298.05mm 291.55

With me so far? Good. Now lets assume I want 15 inch rims which are slightly wider to give me that nice fat look. I'm after a set of 7x15's. First we need to determine the ideal width of tyre for my new wider wheels. 7 inches = 177.8mm. The closest standard tyre width to that is actually 205mm so that's what we'll use. (remember the tyre width is larger than the width of the bead fitting.)

Radius of wheel = 7.5 inches (half of 15) = 190.5mm =203.2

We know that the overall rolling radius must be as close to 298.05mm as possible

So the section height must be 298.05mm-190.5mm = 107.55mm 88.35

Figure out what percentage of 205mm is 107.55mm. In this case it's 52.5%

So combine the figures - the new tyre must be 205/50 R15

.giving a new rolling radius of 293mm - more than close enough.

A tire size calculator.

Current wheel/tyre	New wheel/tyre
145 155 165 175 185 195 205 215 225 230 235 245 255 265 275 285 295 305 315 325 335 / 30 35 40 45 50 55 60 65 70 75 80 85 R 12 13 14 15 15.5 16 16.5 17 18 19 19.5 20 21 22 23	145 155 165 175 185 195 205 215 225 235 245 255 265 275 285 295 305 315 325 335 / 30 35 40 45 50 55 60 65 70 75 80 85 R 12 13 14 15 16 16.5 17 18 19 19.5 20 21 22 23
Current Rolling Radius:mm	New Rolling Radius:mm
Current circumference:mm	New circumference:mm
Difference in circumference:mm or %
So when your speedo reads 70mph, you're actually travelling at mph

The Plus One concept

The plus one concept describes the proper sizing up of a wheel and tyre combo without all that spiel I've gone through above. Basically, each time you add 1 inch to the wheel diameter, add 20mm to the tyre width and subtract 10% from the aspect ratio. This compensates nicely for the increases in rim width that generally accompany increases in diameter too. By using a larger diameter wheel with a lower profile tyre it's possible to properly maintain the overall rolling radius, keeping odometer and speedometer changes negligible. By using a tyre with a shorter sidewall, you gain quickness in steering response and better lateral stability. The visual appeal is obvious, most wheels look better than the sidewall of the tyre, so the more wheel and less sidewall there is, the better it looks.

Tyre size table upto 17' wheels

Here, for those of you who can't or won't calculate your tyre size, is a table of equivalent tyres. These all give rolling radii within a few mm of each other and would mostly be acceptable, depending on the wheel rim size you're after.

Oversizing tyres

If you want the fat look but don't want to go bonkers with new wheels, you can oversize the tyres on the rims usually by about 20mm (to be safe). So if your standard tyres are 185/60 R14s, you can oversize them to about 205mm. But make sure you recalculate the percentage value to keep the sidewall height the same. And finally, you might like to check out this little program written by Brian Cassidy (skyline6969@btinternet.com),which helps with tyre size calculation.

Fat or thin? The question of contact patches and grip.

If there's one question guaranteed to promote argument and counter argument, it's this : do wide tyres give me better grip?
Fat tyres look good. In fact they look stonkingly good. In the dry they are mercilessly full of grip. In the wet, you might want to make sure your insurance is paid up, especially if you're in a rear-wheel-drive car. Contrary to what you might think (and to what I used to think), bigger contact patch does not necessarily mean increased grip. Better yet, fatter tyres do not mean bigger contact patch. Confused? Check it out:

Pressure=weight/area.
That's about as simple a physics equation as you can get. For the general case of most car tyres travelling on a road, it works pretty well. Let me explain. Let's say you've got some regular tyres, as supplied with your car. They're inflated to 30psi and your car weighs 1500Kg. Roughly speaking, each tyre is taking about a quarter of your car's weight - in this case 375Kg. In metric, 30psi is about 2.11Kg/cm . By that formula, the area of your contact patch is going to be roughly 375 / 2.11 = 177.7cm (weight divided by pressure) Let's say your standard tyres are 185/65R14 - a good middle-ground, factory-fit tyre. That means the tread width is 18.5cm side to side. So your contact patch with all these variables is going to be about 177.7cm / 18.5, which is 9.8cm. Your contact patch is a rectangle 18.5cm across the width of the tyre by 9.8cm front-to-back where it sits 'flat' on the road.
Still with me? Great. You've taken your car to the tyre dealer and with the help of my tyre calculator, figured out that you can get some swanky 225/50R15 tyres. You polish up the 15inch rims, get the tyres fitted and drive off. Let's look at the equation again. The weight of your car bearing down on the wheels hasn't changed. The PSI in the tyres is going to be about the same. If those two variables haven't changed, then your contact patch is still going to be the same : 177.7cm ^. However you now have wider tyres - the tread width is now 22.5cm instead of 18.5cm. The same contact patch but with wider tyres means a narrower contact area front-to-back. In this example, it becomes 177.7cm / 22.5, which is 7.8cm.

And there is your 'eureka' moment. Overall, the area of your contact patch has remained more or less identical. But by putting wider tyres on, the shape of the contact patch has changed. Actually, the contact patch is really a squashed oval rather than a rectangle, but for the sake of simplicity on this site, I've illustrated it as a rectangle - it makes the concept a little easier to understand. So has the penny dropped? I'll assume it has. So now you understand that it makes no difference to the contact patch, this leads us on nicely to the sticky topic of grip.
The area of the contact patch does not affect the actual grip of the tyre (strange though it may seem) but it does allow the tyre to distribute heat across the contact patch better, making their operational range greater. The things that affect grip are the coefficient of friction and the load on the tyre. Well we know from above that the load on the tyre remains pretty much the same. Of course it varies in corners as more weight is transferred because of cornering forces, but for the sake of simplicity, load is constant. That leaves on last factor - coefficient of friction. Friction is basically dependant on the rubber compound used to make the tyre, and how that compound changes it's coefficient of friction based on heat. Generally speaking, tyre rubber gets stickier the warmer it gets. At least to operating temperature, then it starts to overheat and it can all go pear-shaped. That's why my comment right at the top about heat dispersal on larger tyres is so important. That's also why Formula-1 teams have tyre-warmers in use before the tyres actually get put on the cars. The rubber compound that's in your tyres is something you'll only be able to find out by calling a tyre dealer, or the manufacturer. But the equation you need to know is simple :
softer rubber = quicker wear = shorter tyre life.
Generally speaking, that's why fatter tyres are generally regarded to have more grip - they're normally made of a softer rubber compound with a higher coefficient of friction. It's nothing to do with contact patch size after all.

I can tell you're still thinking about this. And the question bubbling around your head now is this:
Why doesn't friction depend on surface area?
Well, although a larger area of contact between two surfaces would create a larger source of frictional forces, it also reduces the pressure between the two surfaces for a given force holding them together. In this case, gravity is the force holding your car on the road. As I told you above, pressure = weight / area. So it works out that the increase in friction generating area is exactly offset by the reduction in pressure; the resulting frictional forces, then, are dependent only on the frictional coefficient of the materials and the force holding them together. If you were to increase the force as you increased the area to keep pressure the same, then increasing the area would increase the frictional force between the two surfaces.

In laymans terms : the weight of your car isn't changing - that's the force keeping your tyre pressed against the road. The contact patch area doesn't change - I've explained that above. Your tyre isn't changing it's coefficient of friction (unless something is going badly wrong). To get more grip, you need to increase the force as well as the coefficient of friction. This is exactly what you see in Formula 1. Wings on the car increase the pressure on the tyres as the car goes faster, and the rubber compound in the tyres increases it's coefficient of friction as they get hotter. That equates to massive grip in the corners and ground-hugging speed on the straights. In fact the wings on an F1 car generate so much extra downforce that it more than doubles the weight of the car. In real terms, that means if someone built a racing track upside down, you could race Formula 1 cars on it and they'd stick to the track because the downforce is greater than the weight of the car under gravity. Neat eh?

That last paragraph also explains why dynamic setup on your car is pretty important. All the theory I've gone through so far is based on a static system - the car is driving at a constant speed in a straight line. In reality of course, the contact patch is effectively spinning around your tyre at some horrendous speed. When you brake or corner, load-transfer happens and all the tyres start to behave differently to each other. This is why weight transfer makes such a difference the handling dynamics of the car. Braking for instance; weight moves forward, so load on the front tyres increases. Pressure stays the same, so by your newly-learned formula, the contact patch area must increase. Using a bastardised version of the friction theory, the same load-per-m of contact patch, but more contact patch = more grip. The reverse happens to the rear at the same time, creating a car which can oversteer at the drop of a hat. The Mercedes A-class had this problem when it came out. The load-transfer was all wrong, and a rapid left-right-left on the steering wheel would upset the load so much that the vehicle lost grip in the rear, went sideways, re-acquired grip and rolled over. (That's since been changed.) The Audi TT had a problem too because the load on it's rear wheels wasn't enough to prevent understeer which is why all the new models have that daft little spoiler on the back.

If your brain isn't running out of your ears already, then here's a link to a raging debate that happened in 2000 on one of the Subaru forums about this very subject. If you decide to read this, you should bear in mind that Simon de Banke, webmaster of ScoobyNet, is a highly respected expert in vehicle dynamics and handling, and is also an extremely talented rally driver. It's also worth noting that he holds the World Record for driving sideways..

If you decide to fatten up the tyres on your car, another consideration should be clearance with bits of your car. There's no point in getting super-fat tyres if they're going to rub against the inside of your wheel arches. Also, on cars with McPherson strut front suspension, there's a very real possibility that the tyre will foul the steering linkage on the suspension. Check it first!

Diagnosing problems from tyre wear.

Firstly, let me state my views on rotating your tyres. This is the practice of swapping the front and back tyres to even out the wear. I personally don't think this is a particularly clever thing to do. Think about it: the tyres begin to wear in a pattern, however good or bad, that matches their position on the car. If you now change them all around, you end up with tyres worn for the rear being placed on the front and vice versa. The upside of it, of course, (which many people will tell you) is even overall tyre wear. By this, they mean wear in the tread depth. This is a valid point, but if you can't be bothered to buy a new pair of tyres when the old pair wear too much, then you shouldn't be on the road, let alone kidding yourself that putting worn front tyres on the back and partly worn back tyres on the front will cure your problem. But that's only my point of view.
Your tyre wear pattern can tell you a lot about any problems you might be having with the wheel/tyre/suspension geometry setup. The first two signs to look for are over- and under-inflation. These are relatively easy to spot:

Here's a generic fault-finding table for most types of tyre wear:

Checking your tyres.

It's amazing that so many people pay such scant attention to their tyres. If you're travelling at 70mph on the motorway, four little 20-square-centimetre pads of rubber are all that sits between you and a potential accident. If you don't take care of your tyres, those contact patches will not be doing their job properly. If you're happy with riding around on worn tyres, that's fine, but don't expect them to be of any help if you get into a sticky situation. The key of course, is to check your tyres regularly. If you're a motorcyclist, do it every night before you lock the bike up. For a car, maybe once a week. You're looking for signs of adverse tyres wear (see the section above). You're looking for splits in the tyre sidewall, or chunks of missing rubber gouged out from when you failed to negotatiate that kerb last week. More obvious things to look for are nails sticking out of the tread. Although if you do find something like this, don't pull it out. As long as it's in there, it's sealing the hole. When you pull it out, then you'll get the puncture. That doesn't mean I'm recommending you drive around with a nail in your tyre, but it does mean you can at least get the car to a tyre place to get it pulled out and have the resulting hole plugged. The more you look after your tyres, the more they'll look after you.

Lies, damn lies, and tyre pressure gauges.

Whilst on the subject of checking your tyres, you really ought to check the pressures once every couple of weeks too. Doing this does rather rely on you having, or having access to a working, accurate tyre pressure gauge. If you've got one of those free pencil-type gauges that car dealerships give away free, then I'll pop your bubble right now and tell you it's worth nothing. Same goes for the ones you find on a garage forecourt. Sure they'll fill the tyre with air, but they can be up to 20% out either way. Don't trust them. Frankly, I've yet to find a digital one that works properly, so I steer clear of them too. What you should trust is a decent, branded pressure gauge that you can buy for a small outlay - $30 maybe - and keep it in your glovebox. The best types are the ones housed in a brass casing with a radial display on the front and a pressure relief valve. I keep one in the car all the time and it's interesting to see how badly out the other cheaper or free ones are. My local garage forecourt has an in-line pressure gauge which over-reads by about 1.5psi. This means that if you rely on their gauge, your tyres are all 1.5psi short of their recommended inflation pressure. That's pretty bad. My local garage in England used to have one that under-read by nearly 6 psi, meaning everyone's tyres were rock-hard because they were 6psi over-inflated. I've yet to find one that matches my little calibrated gauge.
One last note : if you're a motorcyclist, don't carry your pressure gauge in your pocket - if you come off, it will tear great chunks of flesh out of you as you careen down the road.

Caster, camber, alignment and other voodoo.

Caster

This is the forward (negative) or backwards (positive) tilt of the spindle steering axis. It is what causes your steering to 'self-centre'. Correct caster is almost always positive. Look at a bicycle - the front forks have a quite obvious rearward tilt to the handlebars, and so are giving positive caster. The whole point of it is to give the car (or bike) a noticeable centrepoint of the steering - a point where it's obvious the car will be going in straight line.

Camber

Camber is the tilt of the top of a wheel inwards or outwards (negative or positive). Proper camber (along with toe and caster) make sure that the tyre tread surface is as flat as possible on the road surface. If your camber is out, you'll get tyre wear. Too much negative camber (wheels tilt inwards) causes tread and tyre wear on the inside edge of the tyre. Consequently, too much positive camber causes wear on the outside edge.
Negative camber is what counteracts the tendancy of the inside wheel during a turn to lean out from the center of the vehicle. 0 or Negative camber is almost always desired. Positive camber would create handling problems.
The technical reason for this is because when the tires on the inside of the turn have negative camber, they will tend to go toward 0 camber, using the contact patch more efficiently during the turn. If the tires had positive camber, during a turn, the inside wheels would tend to even more positive camber, compromising the efficiency of the contact patch because the tyre would effectively only be riding on its outer edge.

Toe in & out

'Toe' is the term given to the left-right alignment of the front wheels relative to each other. Toe-in is where the front edge of the wheels are closer together than the rear, and toe-out is the opposite. Toe-in counteracts the tendancy for the wheels to toe-out under power, like hard acceleration or at motorway speeds (where toe-in disappears). Toe-out counteracts the tendancy for the front wheels to toe-in when turning at motorway speeds. It's all a bit bizarre and contradictory, but it does make a difference. A typical symptom of too much toe-in will be excessive wear and feathering on the outer edges of the tyre tread section. Similarly, too much toe-out will cause the same feathering wear patterns on the inner edges of the tread pattern.

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