97 GTR Tyre data,
Michelin,
Front :- 27 / 65 - 18 .......... Actual dia listed 646mm
Rear :- 30 / 70 - 18 ............ " " " 710mm
Tyre diameter and rolling circumference will vary, due to tyre wear and more important for the rears, wheel rpm at high speed increasing tyre dia which naturally effects the calculated gearing of the car.