is there a chart or some forumula to make sure ur using the right size wheel radius ie if u have a standard alloy on ur car thats 15" with tyres profile at 55, u then get a 17" wheel with a 40mm profile how does it affect ur speedo and even more importantly affect ur diffs on 4 wheel drive,

im not sure if i have worded this question right as its not for me but someone i know building a car up

thanks