Inside the Volkswagen emissions cheating

Typically in the post-OBD2 world there's a Hall Sensor or an optical sensor in the wheel hub and it's counting axle rotation, and then the computer is using some method (who the hell knows if it's just multiplication or something more complex...) to arrive at a number. And then it's displaying a different number.

There certainly *are* GPS speedometers - they're common for kit cars, and google finds plenty of them for sale. It's worth noting those cars don't have to comply with the FCR/NHTSA/FMVSS requirements for a new retail consumer vehicle; they still have to have a speedometer to be street legal to get a VIN assigned, but the rule that says it must reflect a higher-than-measured value at 50MPH, AFAICS, does not apply. Emissions standards are also applied differently in many cases.