The only way to actually accomplish this is to have sensors and recorders on every car all the time and measure it and average it over time (and after the fact).
There's nothing inherently wrong with that, though.
Huh? Nothing wrong with that? Perhaps you might reconsider. At what point do you draw the line on people tracking your activities? Driving? Walking? Brushing teeth? Personal bathroom time? Need I continue?
Most of us are already having our mileage recorded by our insurance company every six months.
What insurance company are you using? I've seen the progressive insurance ads but no one I know has told me about allowing an insurance company to put a GPS tracker in their car for mileage, distance, speed, areas traveled, etc. nor the perks from doing so. There are much better insurance companies than progressive though some might think otherwise because of the snappy commercials.
I just can't see how you would agree to allowing someone to have access to your records unless you are a really boring driver.