Iam going to buy a set of 18 inch wheels for my 1997 a4..how much will that affect the speedometer and odometer/ if any, and how do you change the readings so it will be accurate?

 

it depends on the tire size. whats your stock tire size?

 

Increase or decrease in speedometer reading will be minimal at best. Maybe a half-mile to mile per hour faster. It really doesn't boil down to rim size, but overall circumference.

Even when switching to 18" rims and running 225/40/18 tires, the circumference will generally be about the same as your stock rims and tires. Hence the minimal change.