Wheel & Tire Geometry Calculator

Tire diameter is calculated from the nominal section width, aspect ratio and rim diameter. It is useful for comparing gearing and speedometer effects, but the sidewall label is not a precision measurement. Tire manufacturers publish measured section width and overall diameter for a specific measuring rim.

Wheel position uses nominal bead-seat width and offset. The output shows distance from the hub mounting face to the nominal inner and outer bead-seat edges. It is not physical backspacing because real wheels have flange thickness and lip geometry beyond the nominal width.

A 245/40R17 calculates to roughly 24.7 inches in nominal diameter. If you replace a 24.7-inch reference tire with another 24.7-inch tire, indicated and actual road speed remain essentially the same. A taller tire increases distance traveled per wheel revolution and makes actual speed higher than the speed calculated for the old tire.

This cannot prove S13/S14 clearance. Coilover spring/perch location, brake calipers, tire actual width, camber, caster, steering angle, control arms, fender shape and suspension travel all matter. Use the result for comparison, then measure the actual car and verify the tire manufacturer’s dimensions.