Motorsport Tyre Sizes

[BunkMoreland]

The initial message was deleted from this topic on 02 July 2026 at 20:17

[shirt]

Overall tyre diameter in mm

For a road tyre the aspect ratio changes the diameter for a given rim size, and there is also variation between manufacturers.

This changes the overall effective gear ratio of the car. So giving a defined tyre diameter is more important for a race car.

[InitialDave]

You'll similarly see off road tyres with the diameter stated, for example 33x12.50R15. 33" diameter, 12.50" width, 15" wheel.

[Wh00sher]

With your tyre 255/35R19

Tyre width (mm): 255

Aspect ratio (%): 35 ? sidewall height = 35% of width

Sidewall height = 255×0.35 = 89.25 mm

Wheel diameter (inches): 19 ? convert to mm (1 inch = 25.4 mm)

Wheel diameter = 19 × 25.4 = 482.6 mm

Overall tyre diameter = wheel diameter + 2 × sidewall height

ie 482.6 + (2 × 89.25) = 661.1 mm

Diameter ? 661 mm

Which would be labelled as a 255/660R18