Originally posted by andy v:
It is not the rim size but the overall rolling diameter of the tires. Plus sizing is when you go from a 205/60/15 to a 205/50/16. The rolling diameter is similar due to the shorter sidewall of the tire. I have 225/45/17's on mine and the speedo is only 5% off as the rolling diameter is only SLIGHTLY taller than the stock combo.
That's right. The 5% increase (because of the bigger diameter of the rims/tires) means that if your speedometer indicates 100mph, you are actually rolling at 105mph and that is why the speedometer has to be recalibrated. Same applies to the odometer.
