Originally Posted by Buyimports2

Like I said, the gears in your transmission will not be affected. However, a larger diameter tire than stock will have the same affect as longer gears for the same reason it affects the speedometer. At the same engine rpm and in the same gear, your car will be going faster with larger diameter tires. It thinks it's going the same speed because the tire is spilling spinning at the same rpm, but because the tire is larger, 1 rotation of a larger tire = more ground covered = a higher speed than a smaller tire assuming the gear and engine rpm's stay the same.