The best answer on auto fuel economy

Americans depend on their cars, which are a big source of carbon dioxide. The transportation sector accounts for more than a quarter of U.S. greenhouse gas output. Any effort to combat global warming has to include ways to improve fuel economy, reduce tailpipe emissions, encourage people to drive less or all three.

Under Barack Obama, the Environmental Protection Agency set ambitious standards for fuel efficiency, rising to 36 miles per gallon in real-world driving. But this month, the agency proposed to freeze the number at about 30 in 2020. It argued that the tougher requirement would make vehicles more expensive and less safe while stimulating motorists to drive more—all of which would cause more highway fatalities.

Cutting vehicle weight to save gas is a mixed bag when it comes to safety. Heavier vehicles take longer to stop and are more prone to rollovers. When Ford converted its F-150 pickup from steel to aluminum, saving 700 pounds and increasing its miles per gallon by four, it also got a better crash rating.

The amount of extra driving that better fuel economy would cause is likely to be far less than the EPA claims. Higher mileage standards would, however, save the typical driver hundreds of dollars a year in gas purchases.

The overlooked lesson here is the folly of using federal authority to dictate what sort of cars and trucks are made and purchased. It’s a clumsy and expensive way to reduce carbon emissions.

A better option is one widely recommended by economists: a tax on carbon, collected at the pump, to reflect the harm that emissions cause. This kind of tax would discourage unnecessary driving by making each mile more expensive for all gasoline-powered cars. It would make hybrids and electric vehicles more cost-competitive.

Because of its simplicity and direct impact, it would achieve the biggest bang for the buck. And the burden of the tax—as well as the political risk—could easily be addressed by rebating all the revenue to individual taxpayers.