I have 31's on my Raider and the difference is about 5mph on the speedo, but what do I add for the odometer to make up the tire size difference? Would I add something like 9%? <img src="/forums/images/graemlins/confused.gif" alt="" />

The speedo and odo are run from the same gear train and therefore have the same inaccuracy. Change the speedo drive gear on the t-case end to solve.

Your current tire is 31" in diameter.
Your stock tire was 28.87" in diameter
So... Current tire size divided by stock size = the % over.
31/28.87=1.073... Or 107.3%

To calculate your current mileages simply add 7.3% to your current speedo & odometer readings (multiply them by 1.073).

That said, you are only 5mph faster at ~68mph. All other speeds will be a different amount off.

HTH

Thanks for the numbers. This will help until I get the gears.

I did a lot of odometer correction calculations, using highway mile markers and checking over 10mi runs. I came up with 7.1% correction. Tire rolling radius rarely matches sidewall size exactly, even ignoring inflation factors, but it's a fair approximation.

Eddy, you're likely far more accurate. My calcs are based on 'indicated' tire sizes versus actual tire sizes which are usually smaller and I suspect the error between actual and indicated tire size is greater the larger the tire. <img src="/forums/images/graemlins/cyclops.gif" alt="" />

<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...

I do not know if my speedo gears have been replaced but my fiance and I were pacing each other on the interstate at different speeds giving feedback on walkies and mine is within 1-2 mph at any speed to her speedo. Just curious where would someone get speedometer gears?