Jon70

For example the GT3 cup S has the following:

Michelin race tires 27/65 - 18 for the front
Michelin race tires 31/71 - 18 for the rear

I am guessing the 27 = 270 mm
What is the 65? Is it some alternate value representing the aspect ratio?
18 is obviously the rim diameter.

fc-racer

It's been a while since I ran a full slick, but if memory serves correctly, the numbers are as follows:

27 = 270mm (10.6") tread width
65 = 650mm (25.6") diameter of tire
18 = 18" diameter of wheel

Jon70

Your logic seems correct to me. Thank you.