-alan-
pfm Member
I'm having a little trouble finding the truth behind an assertion I read recently, that any variation in diameter of >3% from the OEM spec of your car, is illegal. Can anybody help? Is this correct, or another of those 'every fule kno' things?
A quick back of the envelope calculation suggests a 3% change in diameter would give you just slightly under a 10% change in the rolling circumference, which would potentially increase your car speedo error beyond the 'nominal' +10%/-0% that manufacturers are believed to be allowed*
Might be just a case that somebody's done the calc. and figured out that's the max change you can have before your speedo is legally unfit for purpose ?
*I haven't had my morning coffee yet, so that calc could be rubbish btw..