It is almost never wise to buy a new car just because you fear prices are about to go up. But if you have been shopping for a fuel-efficient vehicle and are indecisive about whether to pull the trigger now or later this spring, think about getting to a dealership. Today, if you can.

As more car makers feel the pinch of parts shortages in the wake of the earthquake and tsunami in Japan, the supply of vehicles is likely to tighten, severely for certain models. And prices are sure to rise. In some cases the increased cost will...