speed readings are generally very accurate on gps as long as they have a good lock on the sats. eg they are generally MUCH more accurate than car speedometers (zero to +10%) which rely on tyre size, eddy currents, cables etc.

but they can sometimes give some wacky results, like 500mph, when you startup in a new area. however once settled down with a good lock and open skies they are the best

after all the top units are used to measure changes in the earth's surface of less than an inch around volcanoes and fault lines etc