If they did not have other motives, a simple mileage tracker would suffice.
Many new cars also have data recorders similar to aricraft "black boxes". What do you wanna bet that will be potentially used against members of the citizenry?
I think they all do these days (even my 1998 car has one), and they've already been used against drivers. Since they record things like speed, brake and accelerator application, etc., they've been used in prosecuting drivers in traffic accidents.
Sentence needs to be re-worded by dropping the word "potentially".
The Fedgov is out to implement "all-citizen-all-the-time" tracking, by whatever means it takes---national ID cards, national "health-care" databases, vehicle tracking. You name it, and they are out to implement it.
The only reason they aren't going directly to the type that can be physically implanted into our bodies is that the technology to power devices that track continuously isn't available.