In the OBDLink app I'm running, there's an option to adjust the 'vehicle speed scale factor' in the 'Vehicle Editor' section. I have not found a discussion of this part of the app, so sharing here seemed logical.
I'm running 285/75/17 rubber, and the average of the on-paper and in-situ difference in speed indication is 6.5. At the vehicle level, I've set this adjustment to 6% currently (1.06 scale factor) and now the speed from the app (calculated) and GPS speed are nearly the same. I ran two speedo variants on one screen, side by side. Also, *I think* that the MPG reading from the app is now accurate. Best parts: MPG is better looking, and I can get very accurate calculations on how many gallons I need to top off minus two, to protect the Charcoal Canister.
Note: this factor adjustment can also be made on the individual dashboard level inside the app, but based on my experiments, once the vehicle-level adjustment is made, it propagates throughout and applies to everything. I could be wrong here. Still have to set up a three-speedo (adjusted, no 'vehicle level', adjusted at gauge level, and GPS) to fully confirm.
If you have insights into this or do it differently, please do tell.
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