How do you suppose the ECM could see anything other than wheel speed? GPS couldn't possibly be accurate enough for the task at hand. And if you had taller tires installed, you'd be going faster than indicated, but the car still wouldn't know it.
I'd assume that the ECU is getting a very accurate read of wheel speed from the several ABS sensors. In this era of multi axis computer control of vehicle stability, I'd say the manufacturers are very confident of sensor accuracy at an individual wheel level.
GPS speed accuracy would be affected by the visibility of satellites and quality of signal, and most of all the update speed of location readings, but it is accurate enough to guide precision weapons at ballistic speeds, and is widely used in motorsports data analysis. In my experience and from what I've read GPS (and timed mile markers) consistently shows most speedometers to be displaying faster speeds than actual, by amounts outside calculation errors. Typically something like 62-63 at an actual 60 with OEM tire sizes. I have to assume there is a regulatory, liability or marketing motivation to not display the actual speed.
