Is a gas tax a good idea? or a higher CAFE?

Yes, but any one of them won't be enough.We need all of them.

We need a tough automotive Corporate Average Fuel Efficiency (CAFE) standard to provide planning certainty.

We need feebates (fees for low efficiency new cars, rebates on high efficiency cars) or fuel taxes to make people want to buy efficient vehicles, and to properly weight operating costs. Otherwise, buyers don't want to buy them, and car companies have to lose money on small cars to sell them - that means car companies fight CAFE tooth and nail. You even have the perverse effect of low prices making small cars seem low-status.

We need taxes to give buyers of used cars an incentive for to look for efficiency: half of all miles driven are driven by vehicles over 6 years old.

Finally, everyone needs an incentive to drive efficiently.

We need a balanced set of regulations and incentives to prevent or mitigate weird results, like the SUV loophole. It's very much like tax policy - minimize any particular tax, broaden the base, and prevent odd side effects.

Here's the story that started this post.