PolicyGuy:
If the government went for a GPS-based solution, I would expect that a certain proportion of the population would import (completely illegal) GPS jammers to 'blind' the GPS tracker in their vehicle. This has proved to be the case in some parts of the USA, where some truckies are keen to avoid "gummint snooping", according to a report I read some time ago.
The problem with this (apart from the general criminality) is that where a road with any decent amount of such traffic passes close to an airport, it can interrupt GPS coverage for aircraft landing at the airport, and disrupt air traffic.
Presumably just the same as when putting a magnet near a smart meter can be detected as tampering. Your GPS odometer could work out that it had no signal for 30% of the time when the usual user is maybe 1-2%, for example in tunnels and whatnot.