Rear differential # spec.
Team Owner
Joined: Sep 2003
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Perhaps you should do a couple things. With a GPS or "mile-markers" confirm the accuracy of the "odometer". If the odometer checks then you might rely on the GPS to do a confirmation of the "speedometer" for accuracy. I've used a Delorme GPS that did MPH in tenths "reliably"! It's easier to do the two tests "separately".
Now assuming your "tach" is correct you can confirm the speed at a specific RPM and calculate with in reason the axle ratio doing some math.
Here's the math explained and a simple calculator for ease of use. NOTE that the "tire dimension" used is "loaded radius"! You will need to measure that "accurately". There are other calculators that do similar but some/most don't rely on a "loaded radius" dimension. Speedometer calibrations from manufacturers use "loaded radius" for correction. I would do the comparisons in 3d gear which is 1:1.
I like this one because the "math is explained", you can do the math yourself if you've doubts!!
http://wahiduddin.net/calc/calc_speed_rpm.htm
Last edited by WVZR-1; Mar 15, 2009 at 09:42 PM.
