I doubt it's worth messing with, but you COULD add just a bit to the Tire Size. Maybe try 2715, since your speed on the Gryphon is just a bit low. Trouble is, the odometer in the truck seems to be just a tiny bit high, which would say to
reduce the TS.
You really need to test over a long distance (about 100 miles) to get a more accurate comparison (which is what I did on an interstate trip). And, if you want to base the correction on speed, try to get on a straight stretch where you can hold a constant speed for 10-15 seconds. That lets everything stabilize.
Realize too, that your Gryphon MAY have been seeing 55.4 mph and the GPS may have been seeing 55.5-55.6, but due to "rounding" it would look like a full 1 mph difference.
However, I doubt you're going to get much better than what you have now. And yes, by correcting the speed, the part throttle shift points are happening when they should, since they are based on both vehicle speed and throttle opening.
- Jack