and gets out of whack for others when fuel prices swing (because road maintenance cost doesn't swing with fuel prices; and the tax doesn't change, it is just a percent per unit of fuel, regardless of the fuel price).
You can have a fixed tax, let say $0.5/L. Problem solved.
But it doesn't work at all with EV's, and less effectively with HEV's
EVs tend to be subsidized one way or another. So you may remove other subsidies and keep this one (not having to pay for gas tax). With about 1% EV on the road, this is far from a problem anyways.
What we want is for those who drive the most to switch to EV, so that we can have cleaner air.
But we can put a separate yearly distance tax on EV at some point.
Actually it is very simple. Drop the gas/diesel tax completely, and tax based on annually-collected odometer readings instead, adjusted by vehicle weight. Problem solved. Most states collect the readings annually already with vehicle inspection. What I very much oppose is the government forcing people to put spy devices in their vehicles (in any form) that can monitor ANYTHING except distance.
Gas tax is far less easy to cheat compared to odometer reading, however. So as long as there are less than 10% EV on roads, I think gas tax is the best solution.