If you are going from the factory size 255/70/18 on 18" wheel to the factory size of 255/75/17 on 17" wheel then your speedometer should really not be effected. according to the tire calculator on discount tire website the difference in circumference is .02 and says that at 65mph you would be going 65.01mph with the 17" wheels.Thanks, thats exactly what we want. Does it affect the gas mileage or speedomotor?
I'm going through this same calculation for my 2012 Sahara JKU. After investigating different tire sizes and wheel choices ad nauseum, I think I'm going back to reversing the OEM OWL Bridgestone Duelers black-side-out, powder-coating the factory wheels for $140 a pop -- then forgetting about it until the tires wear out! My head hurts.