It's the same with things like tax-by-mile.
This can't, for practical reasons, be decreed overnight. For one thing, most of the cars in service – or at least, a very large number – are not equipped from the factory with the necessary technology.
But, almost all new cars are equipped with the technology. They have some form of the send-and-receive-capable telemetry system, marketed as "concierge" or "driver assistance" technology. GM's OnStar was the first, but pretty much every other car company now has some version of this as well (e.g., Subaru StarLink, Ford MyLink, Chrysler UConnect) and it's rapidly becoming part of the standard equipment suite.
As this stuff becomes de facto standard equipment in cars – Uncle may not even have to mandate it – it'll be much easier for Uncle to demand that drivers be taxed according to mileage rather than by the gallon. It is as inevitable as going gray.
But is it a bad idea?
If you care about preserving the thing that defines not just a free society but a civilized one (they're both the same thing, of course). And that thing is…