Tire Selection for Autocross or Improved Performance
by J.D. Crownover
Love your Alfa’s handling characteristics but wish you could get more? A new set of tires may be the easiest and cheapest way to make you feel like part of the Super Touring Car crowd.
If you want to keep the cost and bother down, simply get another set of tires in the same size you currently have, but with better “gripping power.” There are two tire standards which can help you in your decision.
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. The tires I chose for my Alfa have a wear rating of 160. I could have gotten a set with a rating of 120 but chose mine for their superior rain handling since they would be on the Alfa all the time.
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 (same thing which caused the Mars probe to fail.) 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 (the original tires on my Graduate.)
1) 185 / 25.4 = 7.28 (my tire width in inches).
2) 7.28 * .70 = 5.10
3) 5.10 * 2 = 10.20
4) 10.20 + 14 = 24.20 (my 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, so I suggest you get with somebody experienced with your car and wheel setup to determine how wide of a tire you can use. (Carter Hendricks is one such person and helped with my tire & wheel selection.)
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
