I have a quick question about new tire sizes and the affect on the speedometer. Tomorrow I will be getting 245/40/18's on the front which, according to the tire calculators, when the needle reads 60 I'll be going 60.6 MPH. I will also be getting 255/45/18's on the rear and according to the calculators when it reads 60 I'll be going 59.2 MPH. Am I to assume that this basically evens each other out so that 60 MPH is 60 MPH? Or is the speedometer affected by just the fronts or the rears? Not knowing a lot about this subject, I hope some of you experts can weigh in here. Just want to be able to understand what the true meaning of my speedometer will be now.

 

no no no no

The speedometer reads the rear tires. On a rainy day hit the gas. The rear tires spin and the speedometer moves. As you mentioned, rear tire circumference effects speedometer reading.