Tire Sizes

[Mike325i]

Can someone post a link for a tire calculator that tells you how much your speedo will be off..

mike

[Bob ///M3]

Here you go Mike!

You'll need the old and new tire size (diameter); the final (drive gear) ratio and to input a speed (mph).

Speedometer Error Calculator

Bob ///M3

Originally posted by Mike325i
Can someone post a link for a tire calculator that tells you how much your speedo will be off..

mike

[eurospeed]

Actually you shouldn't need to include any gear ratio whatsoever as the answer is easier than you think. Let's say you go from 225 to 245. Take the 225/40-17 (24.09 in dia) wheel and the 245/40-17 (24.72 in dia) wheel; dividing new by old will result in about a 1.026 increase. This will be the factor your speedo will be off (reduced, so divide by 1.026) because everything else is equal and cancels out. To further explain, the 24.09 dia wheel is rotating 2630.14 times per mile, whereas the 24.72 dia wheel is rotating 2563.11 times. The fewer rotations that the larger tire completes means the speedo indicates a speed inversely proportional to the increase in diameter.