Getting More Mileage Out Of The Gas Tax

Reihan hypes James Whitty's proposal from the Oregon Department of Transportation. Whitty suggests a tax on mileage as an alternative to the gas tax, as cars and trucks become more fuel-efficient:

Whitty’s suggestion that the government allow drivers to choose to buy unlimited driving rights or pay a mileage tax allows drivers to make a bet about how much they’re actually going to drive. If I think I’m going to drive a lot, or if I really hate the idea of the government keeping track of my mileage, I’ll pay the presumably large lump sum. Perhaps I’d have to drive a tremendous amount to justify the expense.

I make a similar calculation every month when I purchase a MetroCard. Should I buy an Unlimited Ride card, and bet that I’ll use the subway at least X times a day on average? Or should I buy a fixed amount, on the assumption that since I’ll be in California for a spell, I will use the subway (X-n) times on average.