The Obama administration, in its analysis of the original rules, concluded that the rebound effect was fairly modest: For every 1 percent increase in vehicle fuel economy, people would drive about 0.1 percent more.

The Trump administration, in its new proposal, reworked that analysis and concluded that the rebound effect was essentially twice as large. People with more efficient cars would drive more miles than previously assumed, and hence were likely to get into many more accidents.

Some economists are questioning the Trump administration’s newer, higher estimate of the rebound effect. Kenneth Gillingham, a Yale economist, points out that the Trump rollback proposal cited one of his papers, which inferred a larger rebound effect from changes in oil prices, but ignored some more recent studies , including one that he led, that found a much smaller effect.

There’s also some evidence, Dr. Gillingham said, that the rebound effect shrinks as Americans get richer, which suggests that this should be less of a problem in the future — an argument that the Trump proposal rejected.

“I think it’s fair to say that their number is at the high end,” Dr. Gillingham said. “And there are several arguments they dismissed that could bring it down.”

The ‘pricier vehicles’ argument

The Trump administration also argues against the Obama-era fuel economy standards by estimating that they would add about $1,900 to the average cost of a new car. That, in turn, will deter people from buying newer vehicles with advanced safety features like automatic emergency braking systems and keep them in older, less-safe vehicles for longer.

The Transportation Department laid some groundwork for this argument in April, when it published a report showing that more vehicle fatalities occurred in older cars than in newer cars. And the agency developed a new model to quantify this effect.