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.