"A consistent 4mph over read over its range" is actually pretty typical.
The EU accuracy spec is as follows:
2.3.7. the speed displayed must never be lower than the actual speed. Between speed V1, read on the speedometer and actual speed V2: there must be the following relationship with the test values specified in item 2.3.5 and between those values:
0 <= (V1 - V2) <= 0,1 · V2 + 4 km/h.
As Sallyann1234 says, manufacturers will always aim for mid range of such a spec, but modern speedometers are remarkably accurate and so cluster quite closely around an overread of about 5 mph.