Quote Originally Posted by gotboost3 View Post
how much would you say an inch throws it off? basically a 1/2 inch all the way around? how do you calibrate you speedo?
Go here and put in the original stock tire size and your new tire size. It tells you all the differences:

Tire size calculator

For calibrating the speedo, you just take info from that page and someone with a tuner adjusts the "pulses per mile" value in your PCM.