Really?
I’ve used the MPH shown on my Garmin GPS verses the actual speedometer on the car.
The GPS is more accurate in acutal miles per hour.
YMMV.
I’m saying actually that my GPS somehow picks up the posted speed limit and compares it to my speed and my speed flashes red to indicate that I’m speeding. The inaccuracy is in the determination of the posted speed limit.
I’ve been on roads where the speed limit is 75mph, but the GPS thinks it is 65mph.