no way a gps and mechanical are going to get the exact speed. but I have noticed my bosch's tend to be about 1.5 mph slower. gps often jump around so its hard to tell and the gps may take longer to start and stop.
That's the way it will be handled. Not my choice of method. Yes, support up to 20 mpg is what I paid for. They have the tire size max size set to a tire that would be 32.25 inches tall. That's taller than the fenders.