20"/432mm

The sizing of racing chair tyres is peculiar...please check your wheel size before ordering.

Although called 20" tyres, this size has a measured diameter: 

432mm + (2 x 18mm) = 468mm / 18.4"

If your wheel + tyre combination measures just over 18" in diameter, you're in the right place. If it's just over 16", look at the 18" tyre section....don't ask, it's complicated...