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Calculation of tire diameters

Wheels and Tires

Vehicle Information: Year: 2000; Body Type: Coupe; Total Mileage: 21,000;

Would you please explain the formula you use for calculating tire diameters? I have a 2000 C4 that I am currently running 225/40-18's in the front, and 285/30-18's in the rear. I am using the standard factory 2002 5 spoke superlight alloys (8" front, and 10" rear). As I understand it, I am well within the limits of the 4% factory limit for the safety of the differential and the AWD system. I am in the market for replacing the tires soon and I was interested in replacing the front tires with a slightly wider tire to reduce understeer. I currently have Bilstein PSS9 coilovers installed and I have just under 1/2" of space between the tire and the spring. Will 235/35 - 18's fit in the front and will the tire size ratio be acceptable for the 4% rule? Thanks for your help in advance.

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