Which

High taxes, which would piss off voters, but give them a choice in what to buy, or tell the manufacturers what to build, limiting choice.

I'd vote for the taxes ... and side benefit would be funds for road repairs

How Will Obama and Detroit Get Americans to Buy Hybrid Vehicles? - WSJ.com:

"U.S. car makers have lobbied for higher gas taxes as the simplest way to push consumers into high-mileage cars. The Obama administration is betting on a different approach: Leave gas taxes alone, and instead invest government money in advanced battery development, offer tax breaks of up to $7,500 on hybrids and mandate tougher mileage standards to force car makers to use new fuel-saving technology. Washington now has a big financial stake in getting this right, or billions in public money plowed into Chrysler and GM could be vulnerable to energy markets."