THAT was MY first thought....we ALREADY pay taxes on GAS for this VERY reason.....they really do think we are stupid.
As motor vehicles become more efficient, the revenue from gasoline taxes decreases. Let's say you buy a gallon of gasoline for $4.00 of which $2.00 is tax. That's $0.05 per mile in a Prius as opposed to $0.20 per mile in a Suburban.