As stated above, rim diameter doesn't play much part in speedometer readings, its the outer tire diameter that matters. Use the calculators at TireRack.com or any of the other online ones. Is the speedometer accurate now? How about the odometer? Its better to set the odometer since it is gear driven right off the cable. The speedometer is magnetically driven and over time can loose magnetic strength thus showing slower then actual speeds and the faster you go, the farther off it gets.
Find a marked section of highway and drive a good 10 miles each direction and see what your odometer shows, then with the new tires do the same thing. Shorter tires will add miles, taller tires will be less. It works just like a short legged dog running with a tall dog, the short one will take a whole lot more steps!
Joe
