the whole idea is foolish - if the complaint is that greater fuel efficiency lowers the revenue, just raise the gas tax. why attempt to install some massive sensor based mileage collection system everywhere - just raise the friggin' gas tax 5 cents a gallon and be done with it.
You're thinking too intelligently, not politics and greed. Politics because raising the gas tax is a big no-no. Greed because that raise would be again undercut by the next generation of fuel efficient cars. Remember, they don't really care about fuel efficiency, they just want their money, but they can't say that or the eco-weenies would kill them.
Silly, it is not about the tax, it is about being able to track your every movement. The tax is the excuse.