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Old 11-09-2013, 12:13 AM
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Rzeppa Rzeppa is offline
Cruise Moab Chairman
Join Date: Aug 2005
Location: Kittredge CO, USA
Posts: 6,727

As a former instrumentation designer who did a lot of work with NIST and DFM, I advise that you need to ditch your tire diameter calculations and go with real-world documented measurements when you are doing your compensation, and go with the best resolution that is relevant to your calculations.

What I do to calculate my MPG corrections for tire size is write down my odometer reading as I pass mile marker 100 (just west of Glenwood) on I-70 on my way to Moab. Then when I cross the Utah line I write down the odometer reading. That gives me my real-world compensation coefficient to better than 1% resolution.

Other people have written that their GPS works also. YMMV, but simply going by tire diameter neglects numerous sources of error.
Jeff Zepp
Kittredge CO USA
1971 & 1976 FJ40s, 1978 FJ45, 1987 FJ60
Rising Sun 4WD Club, TLCA #4063
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