I may be wrong but as I understand it, DOT requires that all speedometers on vehicles in the US of A must be either right on, or optimistic.
They can never say you are going slower than you actually are.
To meet this requirement most of the speedometers read 0 to 5% above the actual speed that's being traveled.
Some speedometers will read up to 10 percent above the actual speed so an indicated 70 might only be 63 actual and an indicated 80 might be 72 but at a 5 percent error an indicated 70 would be 66.5 mph and an indicated 80 would be 76 mph.
As for my G5's speedometer it seems to be from 1 to 3 mph higher than I'm going if I'm riding at a speedometer reading of 45 mph, at least according to the radar speed signs that flash your speed on a big black panal.
They have these things sitting alongside the road around here.
To my way of thinking, that's close enough, especially for a old fashioned mechanical device that is relying on tire size, tire inflation and whirling magnets to make it work.