Posted by bdk on Tuesday, December 12, 2000 at 15:40:08:
In Reply to: HOW DOES TIRE SIZE AFFECT SPEEDOMETER posted by FRANK on Monday, December 11, 2000 at 19:41:51:
Actually, tire size has no effect whatsoever on the speedometer...since the speedometer really measures RPM and is calibrated to display roadspeed.
What you want is the speedometer correction factor required for different tire sizes.
Break out the calculator and try this: Speedometer reading X new tire diameter / original (as delivered from the factory) tire diameter = actual speed.
With a larger "new tire diameter," you will be traveling faster than the speedometer says you are. Conversely, with a smaller "new tire diameter," you will be traveling slower than the speedometer says you are.
Example: Factory original tire diameter = 34 inches. New tire diameter = 36 inches.
55 mph indicated X 36/34 = 58.2 mph actual speed.
This calculation also works if you use the tire radius instead of the diameter.
I've had good luck finding tire diameters on tire manufacturer/distributor/dealer websites.
Let's hear it for mathematics!