Seems to me gasoline taxes are the fairest way, for several reasons. First, gas taxes are used in large part (or they should be) to maintain roads. Based on the laws of physics, heavier vehicles cause more road wear than lighter vehicles. Heavier vehicles also consume more gas, therefore, taxing gasoline consumption is (at least in first order approximation) taxing the actual impact a vehicle has. Second, it allows people to control how much tax they pay, at least to some extent. With a mileage tax, someone who needs to drive a large number of miles (for their job or whatever reason) has no choice but to either pay the tax or reduce the number of miles they drive. With a gasoline tax, the driver can trade an SUV that gets 20 miles per gallon on the highway for a car that gets 40, thus allowing him to drive twice as many miles for the same tax. Third, by its very nature, a tax on gasoline encourages both conservation and "green" technology. Conserving energy makes us less dependent on other nations for oil, and "green" technology creates (or at least has a lot better chance of doing so than does importing oil) jobs in this country.
Don't get me wrong. Having a per-mile tax on driving is better than doing nothing. But we can kill a lot more birds with the same stone if we instead have new gasoline taxes.
Bettles