Interesting. I'm a big advocate of turning the gas tax into a percentage tax, rather the stupid flat fee per gallon. The flat fee model requires politicians to continually "raise" the gas tax in order to keep up with inflation, which is a politically damaging move on their part.
In contrast, a percentage tax would not lose value over time, although the revenue might decline in real terms if gains in fuel efficiency cancel out the growth in fuel use.
US Federal Fuel Tax, Historically
I'd love to see somebody make a graph of fuel tax receipts, adjusted for inflation, and overlaid with the number of registered vehicles. That would provide a clearer picture of the huge funding gap that's causing our roads, bridges, and train lines to fall apart.