Driving levels started creeping back up as soon as gas prices started receding, but a gas tax would be permanent and would lead to even bigger changes in habits. And the cost is lower than it seems. Economists point out that the energy savings would not change if the government returned all the revenue raised by a gas tax to Americans — perhaps through rebates for low-income people who spend a bigger share of their money on gas.

The weakness with the fuel-economy rules is that they don’t affect people’s behavior the way higher gas prices do. They apply only to new vehicles — not the ones on the road now — so it takes quite a long time to alter our overall gas use. And they carry perverse incentives: because new vehicles go farther on a gallon of gas, they give us a reason to drive more, leading to more congestion, accidents, pollution and gas consumption.

The incentives to carmakers can also be weird. The original standards for fuel economy in the 1970s exempted light trucks, which were a small share of the market. That decision was critical to the explosive growth of the S.U.V. In 1973, light trucks amounted to 3 percent of new vehicle sales. Today they account for half.

Who knows what distortions the new rules will bring? The standards vary according to the footprint of the car — the length between the axles multiplied by the width. So maybe cars will be boxier in the future.

Automakers will make the most efficient cars they can that customers will buy. A gas tax that goads drivers to choose gas-sippers takes advantage of this fact. A mileage standard does not.

Christopher Knittel, an energy economist at the Massachusetts Institute of Technology, estimated that if carmakers had devoted all their technological progress since 1980 to improving fuel efficiency, gas mileage would have improved 60 percent by 2006. Instead, they put most of their effort into more power and weight, and fuel economy gained less than 12 percent.

All this makes mileage standards an expensive way to restrain our energy use.

According to the government’s analysis, the additional production and maintenance costs made necessary by the mileage rules will rise gradually to about $31.7 billion in 2025 — which will add about $1,900 to the average price of cars and light trucks. There are other costs, too. Some Americans will not be able to afford a new car. Profits of some automakers and dealers are likely to decline. Greater congestion will impose an added burden on health.