What a gas tax has going for it is that it is solution-neutral, and least disruptive of the market for alternative energy than anything else. Take a tax break for hybrid cars - that pushes people in favor of one specific technology, at the expense of others (such as clean diesel or ethanol). The same goes for tax breaks for ethanol production, or other attempts to micro-manage the energy market.
Governments are horrible at picking winning and losing technologies. They should not meddle in what the market does best. So if you need to change behaviour on a large scale, the best way to do that is a flat tax universally applied, and then to sit back and let the market figure out how to adjust to the new economic reality.
The same can be said for CAFE standards and speed limits designed to save fuel. CAFE standards have distorted the market by pushing people into light trucks and SUVs. Low speed limits get ignored, and have unintended consequences.
Get rid of them all. Open up the transportation market and let people choose what they want. However, institute a carbon tax. When you buy a vehicle, its carbon emission is rated. Then you report mileage each year on your tax form (subject to audit like any other receipt), and pay taxes on the carbon you pumped into the atmosphere. Now you’re free to lower your taxes by driving less, or driving a smaller vehicle, or going with an alternative technology like electric or a hybrid gas/electric vehicle.
Of course, the problem is that by the time such a bill wound its way through the governmnent, it would have so many exceptions tacked onto it that it would be hellishly complex to administer. But in concept, I believe that’s the right way to approach the problem.