Better (but less simple) idea Get an MPG benchmark, say 25. Give a tax credit for cars that overperform and a tax debit for cars that underperform. This create

Better (but less simple) idea Get an MPG benchmark, say 25. Give a tax credit for cars that overperform and a tax debit for cars that underperform. This creates a revenue-neutral incentive structure that would help cycle in more efficient cars into the market. Every so often, raise the benchmark. Of course you have the problem with fleet vehicles which are always going to be below the benchmark, but they could rotate into natty as a source.