Track-bound Porsche may do well with 18-inch wheels

John Mahler

SPECIAL TO THE STAR

Q: I am planning on doing some track events this year with my 1999 Porsche C2. I'm currently running P225/45R17 and P255/40R17 Michelin PS2's. A great tire.

I'm thinking of going to R-compound like Michelin Pilot Cup Sport. However, the rear tires only come in a P275/40R17.

Can I move from a P255/40R17 to a P275/40R17 tire and still use the same size fronts?

A: It would not be wise to go to a P275 rear tire while using the regular P225 front. The stagger in size between the front and rear is what gives your Porsche the exact handling the factory intended. By going to a bigger rear tire, you would increase the car's understeer and diminish the handling.

There is a much bigger selection of sizes if you move to 18-inch wheels. Plus the 18-inchers will require lower profile tires, which will also improve handling even if you don't switch to R-compounds. When you look for 18-inch wheels for track use, buy the lightest weight possible in a forged wheel. Forged wheels are stronger and are less likely to break if you hit a curb, than cast wheels.

The correct 18-inch upgrade for your '99 C2 Porsche is: front, P225/40R18 and rear P265/35R18. In the Michelin Pilot Sport Cup, these size fronts can go on rims from 7.5 to 9 inches wide and the rears can fit rims from 9 to 10.5 inches. The ideal width for this size tire is front 8 inches and rear 9.5 inches. The most common setup in rim width for this car is 8 inches wide at the front and 10 inches at the rear. That fits in well with these tire sizes.

Some sample weights for high quality, strong, forged Champion RG5 wheels are: 8 by 18 inches, 18 lbs. 5 oz. and 10 by 18 inches, 18 lbs. 13 oz.

Q: Your articles on tires are always very informative. This past winter there have been many questions about tire sizes, for example, going from a 16-inch wheel to a 15-inch wheel for winter tires, etc.

You quite frequently give the tire diameter for different size tires. Can you give us the mathematical formula to calculate the diameter of a tire so we can do it for ourselves?

A: This is how you calculate the tire diameter. Using an example of a tire size of P225/60R16. The 225 is the width of the tire in millimetres. The 60 is the aspect ratio in per cent. The 16 is the wheel diameter in inches. So, multiply the tire width times the aspect ratio by two, divide by 25.4, then add wheel diameter.

Here's how it would work: 225 times 0.6 (because it is a percentage) times 2 (because you have two sidewalls, top and bottom) gives you a result in millimetres. So now divide by 25.4 that gives a result in inches for the sidewall heights. Now you add the wheel diameter in inches for an overall diameter in inches.

Q: I have four spare Bridgestone Potenza Tires on 17-inch aluminum alloy wheels.

The size is P215/ 50R17, 90H, M+S, from a 2008 Toyota Matrix XR. I now have a 2009 Toyota Matrix XRS with BF Goodrich tires, P215/45ZR18, 89W, M+S. Can I use the four spare tires on my 2009 Matrix?

A: Your old tires will work on your new car, The difference in diameter is less than a tenth of an inch between the two. Test-fit a front wheel to make sure the older wheels will clear the brake callipers. It should not be an issue.

Email tire questions to John Mahler at thetireguy_1@hotmail.com. Please include vehicle's make, model and year, tire brand and size, plus your name, address and phone. Volume of mail prevents us from answering all queries or providing personal replies.

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