How much change does larger wheels affect the Odometer. I went from 17's with stock 235 tires to 18's with 255 (F) and 285 (R) and my own test driving one mile on cruise control at 60 Mph came to minute and 2 seconds. I tried this a few times and it was always 2 seconds over the minute. Assuming this is correct how much odometer difference +/- does that mean ???????

 

The stock 235/55/17s have a 27.2" diameter while the 255/45/18 and 285/40/18s have a 27" diameter. What does this mean? It means that when your speedometer is now reading 60 MPH, you are actually going 59.6 MPH.

 

The change in wheel size has NO bearing on the change in the speedometer. It's all about tire OD. Use any of the online tire size calculators to input your OE tire and new tire specs and it'll give you all the details. Typically with a larger wheel you end up with a tire with less sidewall and equivalent tire OD in order to maintain reasonable speedometer inaccuracy.

 

What sidewall?

 

There's no way to tell the difference based on the information you gave, the aspect ratio for the rear tires is needed to figure out the difference. (eg. 235/55R17, with he bold text being the aspect ratio.)

To figure the tire height of the example given above you take the wheel size (18") and add it to double the sidewall, you figure out the sidewall by converting the tire width from millimeters to inches (255 divided by 25.4 = 10.04), the aspect ratio is a percent of the width, so 55% of 10.04 (10.04*0.55= 5.52), you double that sidewall height (5.52*2=11.04), add that to the wheel size (11.04+17=28.04) and you get 28.04".