I agree with everything except the direction of the tire size change. To make the Gryphon (and your dashboard speedo) read slower, which is what I think you want, you need to reduce the Tire Size. The 2 mph difference is probably about the 3% that ticopowell mentioned that compensates for "squish". Try multiplying your current Tire Size by 0.97 and use that result as your TS.
Remember, when you get your Gryphon and GPS to agree, the dash speedo will still read about 2 mph high at 70 mph due to mechanical errors. But, the odometer will be correct.
- Jack
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2014 F150 Platinum SCrew 3.5L EcoBoost 4x4 with SCT programmer
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