While fuel efficiency and emissions for new vehicles are still substantially better than in 2007, both have begun to worsen.

Because vehicles are typically used for many years, current purchases have only a gradual effect on the fuel efficiency of the overall vehicle fleet of new and old cars in the United States. But history shows that fuel efficiency responds to gas prices, among other factors. It is striking, for example, that actual on-road fuel efficiency of the average car declined to 13.4 m.p.g. in 1973 from 15.3 m.p.g. in 1936, according to a paper by Professor Sivak and a colleague at the University of Michigan, Brandon Schoettle.

Most of the progress in fuel economy occurred after the oil embargo of 1973 and in the 1980s. Fuel economy rose to 21.2 m.p.g. by 1991, propelled partly by soaring prices and by Corporate Average Fuel Economy standards, which went into effect for the first time in 1978. After that swift progress, however, on-road fuel economy rose only modestly to 23.4 m.p.g. by 2013, the most recent figure available for the overall American car fleet. The data on fuel efficiency for newly purchased cars suggests that the gasoline price declines of recent months may slow down improvements in fuel economy for the overall fleet at the very moment when reducing dependence on fossil fuels may be becoming more urgent.

To cut back on gasoline consumption, after all, the Obama administration has been counting on higher fuel economy standards, which are likely to be more palatable when fuel costs are high.

One way of increasing the prices paid by consumers at the pump is to tax carbon emissions or, alternatively, to raise the tax on gasoline. These methods don’t reward companies that extract and refine fossil fuel. Many economists favor such taxes. But a carbon tax, which the Obama administration has advocated but Congress has opposed, has failed to gain traction in the United States. And this month, Congress decided to hold the federal gasoline tax at 18.4 cents, the level at which it has been stuck since 1993. Still, as I pointed out in a column a year ago, a gasoline tax could be useful not only as an environmental measure but in rebuilding the nation’s roads and bridges.

In a new paper, Stan A. Kaplowitz and Aaron M. McCright, sociologists at Michigan State University, analyzed public views on raising the gasoline tax. That wouldn’t be a wildly popular move among voters, Professor Kaplowitz said in an interview, but it would be more acceptable under certain circumstances. Not surprisingly, imposing a smaller increase — say, 20 cents a gallon — would be more popular than a $1 increase, for example. And when people are told that the money is to be used for a concrete purpose — for repairing transportation infrastructure, for example — they respond more favorably than when the tax revenues are simply to go into the federal Treasury. A so-called revenue neutral tax, in which people are refunded gas tax money in paychecks or tax returns, would also be more favorably received.

“We found that a gas tax is salable if you tell people the reasons for doing it,” he said. “Raising taxes isn’t popular, but this is not as hopeless as it might seem.”

The problem remains, however, that almost no one likes paying higher taxes, and even drivers who care about the environment really like low gasoline prices. I know I do. On a sunny day, when the traffic is light and gas is cheaper than it’s been in years, I want to get out on the road. Perhaps I should fight the impulse. It might be easier if the true cost of driving were reflected in the price at the pump.