Treadwear
The treadwear rating simply indicates how fast a tire wears in comparison to a government standard. A treadwear rating of 300, for example, means the tire lasted 3 times longer than the government standard. These ratings are not always shown on the tires so you may have to ask what they are. If you’re running a standard tire with a 50-60K warranty you probably have a wear rating of 300-500.
Generally, the lower the treadwear rating the softer the rubber compound and therefore the better they are for cornering.
Traction Rating
There is also a traction rating available. This represents the straight-a-head web braking traction. The ratings are AA best), A, B, & C. Obviously, you want the best you can get and afford.
The next step is to change the size of the tire, which takes some mathematics and possibly some advice from knowledgeable friends or mechanics. But first you need to learn to read "tire".
Tire markings
Tire markings on the side of tires provide a wealth of information, IF you know how to read them. Look at the sidewall of your tire and you'll see a series of numbers like 185/70R14. This indicates your tire and wheel size and is a mix of metric and English measurements. What this is telling you is:
* 185 is the width of your tire in millimeters at its widest point;
* 70 is the ratio of your tire sidewall height (the distance from the tire tread to the wheel rim) to the tire width in percentage;
* R indicates a radial tire
* 14 is your wheel diameter in inches.
Computing tire diameter
You use these numbers to compute the actual tire diameter on your car. You need this when you go to change your tires to ensure your new tire has the same diameter as your original. This is important in keeping your speedometer / odometer correct (it also affects ground clearance.)
1) Divide the first number by 25.4 to get the tire width in inches. This is the width at the sidewall, not the tread width that contacts the ground;
2) Multiply it by the aspect ratio. Remember that it's a percentage so you multiply by a hundredth of the number indicated (just stick a decimal point in front of the number.) This gives you the height of the sidewall. Since a tire is circular, you have two of these in the diameter, so
3) Multiply by 2. You now have the width of the tire as though there was no hole in it, but of course it does have the hole for your wheel, so now you need to
4) Add the wheel diameter to your results. You now have your tire diameter.
Example: 185/70R14
1) 185 / 25.4 = 7.28 (example tire width in inches).
2) 7.28 * .70 = 5.10
3) 5.10 * 2 = 10.20
4) 10.20 + 14 = 24.20 (example tire diameter in inches).
Going to a wider tire
Increasing tire width means putting more rubber on the ground to grip the road. There are things you need to consider, such as wheel width and wheel-well clearance which is too involved to write about here. An experienced or professional is recommended to help find out how wide a tire you can use.
Once you've determined the width tire you want you can use the above formula to compute the numbers you need, either by sticking in numbers from a list of tire choices, or by using parts of the formula in a different order.
I want to try to reach the 24.20 number in my original computation. Running the formula backwards:
4) 24.20 – 14 = 10.20 (Subtract desired tire diameter from wheel diameter)
3) 10.20 / 2 = 5.10 (Gets the sidewall height)
2) 5.10 / 7.50 = .68 (the tire ratio)
1) 7.50 * 25.4 = 190.5 (the tire width in millimeters)
So the perfect tire size would be 190.5/68R14. But tire width and ratios only come in multiples of 5, so you’re going to have to round both figures, and then run the complete formula to see which comes closer to my goal of 24.20"
190/70R14 = 190/25.4 = 7.48 * .70 = 5.24 * 2 = 10.48 + 14 = 24.48
190/65R14 = 190/25.4 = 7.48 * .65 = 4.86 * 2 = 9.72 + 14 = 23.72
Between these two I would take the first since it’s closest to the original tire diameter.
Plus sizing
There are two advantages to going to a wider tire:
1) There is more tire contacting the road and
2) A lower aspect ratio means the tire will flex less in the turns thereby providing better stability.
Plus sizing takes advantage of the latter by using a larger wheel than the original. This means less sidewall and more stability. (It can also mean a harsher ride as there is less sidewall to absorb the bumps.) Just be sure to use the formula to find the correct tire size.
Of course, as long as you’re getting new wheels anyway you might as well get wider ones too, if your car can take it.
There are even more things to consider when looking for a new wheel though: bolt pattern and spacing and a thing called offset. Get expert advice before choosing a wheel which isn’t specified as fitting your car.
Autocross Tires
Of course, if you plan on doing any serious autocrossing you'll want the special tires made especially for this sport. These tires have some or all of the following features:
Soft compound - Autocross tires are super-sticky to get you around those corners. They tend to have a tire wear rating of only 30 - 40.
Tread - Contrary to what you might think, autocross tires are usually not "slicks" (tires with no tread). This is because most autocross classes (a group of similar performance cars competing against each other) require street legal tires. However, they do have a special tread design which puts more of the tire in contact with the road than normal street tires. Because of this they are generally very poor tires in heavy rain (because of hydroplaning) or snow (little tread to grip the snow and the cold prevents them from getting sticky).
Tread depth on an autocross tire is extremely shallow -- legal, but shallow -- another reason for the low wear rating. The shallow tread depth reduces tire "squiggle", the bending of the tire at the tread.
Sidewalls - Autocross tires tend to have stiffer sidewalls, to reduce tire flex. Some even has one sidewall stiffer than the other. The stiffer sidewall is on the outside so in a turn, the tire on the outside of the turn stays upright for stability while the tire on the inside of the turn flexes to keep more of the tire in contact with the surface.
Autocross Vs Racing Tires
You might think that racing tires would make good autocross tires, but this is not the case. Racing tires use a different compound rubber than autocross tires.
In autocrossing you wait in line to run the course and then WHAM, off you go. There's no warm-up permitted, so the tires have to get to operating temperature as quickly as possible. They don't have to operate at this temperature very long -- 30-90 seconds is a typical run -- so a soft rubber compound can be utilized.
Racing tires, on the other hand, have to operate at high temperatures for a much longer period of time and so are built to accommodate that requirement. They get as sticky as autocross tires, once they're warmed up, but it takes much longer to get them to operating temperatures. You've probably heard a NASCAR or CART announcer say that the tires take a lap or two until they are totally effective.
Wrapup
So now that you know a bit buying tires go to your favorite internet computer and surf the web for your perfect tire. There are online tire stores with a much bigger selection than your local tire shop.
by J.D. Crownover
Where can you buy Autocross tires you ask.
HERE
Some Autocross tires to look for:
Hoosier A3S03
Hoosier Autocrosser
Hoosier R3S03
Kumho ECSTA R700
Kumho Ecsta V700
Kumho VictoRacer V700
Michelin Pilot Sport Cup
Pirelli PZero Corsa
Yokohama A032R