Plenty of policy types like the idea of expensive gasoline, which they hope will force America to change its energy habits. Detroit is less thrilled with $4 gas, but resigned to it all the same. The question is: What’s the best way out for U.S. automakers and the environment? To tweak the machine, or build a new one altogether?

Ford Motor Co. and General Motors are both struggling these days, stuck with thousands of gas-guzzling trucks on lots and factories geared to make even more, just as consumers turn their back on big,...