Tire Size / Speedometer Error Calculator
http://www.celicahobby.com/tech/speedcalc.html

This calculator is for comparing original equipment tire size to alternate after market tire sizes. All calculations are round up. Calculations are to be treated as approximate only, due to variations in how tires are manufactured.

The following is an example to help to decode a tire size like P195/60R15
195 = tire width in millimeters, measured from sidewall to sidewall
60 = sidewall ratio, the ratio of sidewall height to tire width
15 = wheel rim diameter in inches


Factory stock Celica GT tires:
P195/60 R15 on steel rims

Factory stock Celica GT-S tires:
P205/55 R15 on aluminum alloy rims
or
P205/50 R16 on aluminum alloy rims


Online Wheel Speed Calibration calculator is at http://www.celicahobby.com/tech/speedcalc.html