As a result, Mr. Bennett said, a carmaker may continue to sell gas guzzlers, offsetting their emissions by selling more low-emission hybrids.

So if carmakers sold more high-mileage hybrids, they could also sell ever-bigger gas guzzlers and still meet the target.

Some who oppose capping the tax credit have said it is a way for Congress to make Japanese hybrids less appealing to consumers as American automakers begin selling more hybrids.

Automakers have an incentive to sell hybrids in California and the Northeast, where stringent emissions regulations are due to be phased in.

Toyota and Honda already sell hybrids.

G.M. and the Ford Motor Company have said they will eventually sell hybrids.

Right now Toyota does not sell plug-in hybrids.

The Prius nameplate has sold 3.67 million hybrids and plug-in hybrids through March 2013.

The company also markets and sells hybrids or improved varieties of sorghum, sunflower, soybean, alfalfa, canola, rice and wheat, as well as forage and grain additives.

Until then, car companies could comply by selling hybrids, efficient gas cars or cars that use alternative fuels.