And to add, the GPS mph isn't always accurate for moment by moment due to elevation changes and slight miscalculations. The misscalculations are very common for standard GPSs that are accurate up to 7 or so feet at the most and on average to 20 ft. Also my gps says my max speed is 371 mph...I wish.
But one thing I'm not seeing is what your intial tire size was. If you have the 2758 programmed in, drive another 20 and redo the calculation. It usually takes a few tries because of rounding errors in the programmer. But if your getting speeding tickets for the speedo being 2 off of the GPS then you are just going a bit too fast.