<img src="/forums/images/graemlins/smile.gif" alt="" /> I don't know about how accurate I am. I used mile marker posts, and had to watch a moving odometer tenths wheel and a mile marker, and count seconds at a steady speed until the wheel reached dead on a number, then count seconds again on the other end after passing the tenth marker. AND, the mile marker posts were set by the GADOT (no one who has ever driven the I85/I285 interchange in NE ATL will wonder why I question a GADOT action), who may have decided the dirt was too hard to drive the post here, or not shady enough, so they went another 100' up the road and set the post, or they just plain goofed.

The best way to do this is to measure from the center of the hub to the ground, sitting level on hard surface, of the old rear tire, truck normally laden. Then repeat with the new tire. This gives the best approximation of rolling radii, with the only source of error other than measurement error being the differing rates of increase of that rolling radius at speed due to centrifugal force. Some tires get taller faster as they spin than others, due to differing construction and weight of tread.

The above gives you two radii. Divide original into new, and if the new tire is taller, you should get a number like 1.06...

To correct mph or miles run, multiply. To find the tooth count of the new speedo driven gear, divide the 1.0x number into the tooth count of the old gear. If the old count is 27 and our ratio of radii is 1.073401841, you would want a new gear with 25.153694 teeth. Since that silly fractional tooth will break off, you get a 25 tooth driven gear. That still leaves you with about a .6% error.

To get rid of that last error, search for a post on speedometer correction from about 3-4 years ago by ??? DougB ??? maybe, that details how to do the rest of the correction inside the speedo.

Congratulations. You now have a calibrated speedometer and may write tickets for speeding based on pacing the miscreant...