The thing about it is......

The tires are only going to change the relationship between what the PCM thinks the vehicle speed is/was and how fast the truck is actually moving.

As long as nothing else changed when you put on the larger tires (they're barely larger than stock as far as revolutions per mile goes anyway), then your truck will shift to 4th at exactly the same speed (as indicated on your speedometer) as before. With the slightly larger tires, your truck is going at a slightly slower speed than it was with the original tires at any given speedometer reading.

As far as comparing your speedometer to a GPS reading, you're not going to get too much more accurate than that.