The Environmental Protection Agency on Thursday announced a plan to weaken fuel-economy standards put in place under the Obama administration.

The new plan would freeze standards at 2020 levels through 2025 and beyond. Under the Obama administration, automakers were required to reach a fleetwide average fuel economy for all cars and light trucks of 51.4 miles per gallon by 2025. The Trump administration’s proposal instead freezes that figure at its 2020 level of 36.9 miles per gallon.

But the EPA’s standards refer to testing under laboratory conditions, not the mileage we get when driving our cars on the road. To know how the new standards would affect real-world gas mileage, researchers need to take the further step of modeling how real-world use differs from laboratory conditions.

Simon Mui, a senior scientist at the Natural Resources Defense Council, a group that opposes the change to the fuel standards, calculates that the change would have the net effect of reducing the average real-world fuel economy of American automobiles by about eight miles per gallon in 2025 relative to what it would be if Obama-era standards were kept in place. Mui assumes that real-world driving would result in about 80 percent of the fuel efficiency measured in laboratory testing. He cautions that as the EPA provides more information about its new proposed standard, those calculations may change.

Under the Obama rule, passenger cars and light trucks would see a real-world average of 37.4 miles per gallon by 2025, according to Mui’s calculations. Under the Trump administration’s preferred proposal, that would drop to 29.6 miles per gallon, a reduction in nationwide fuel efficiency of about 21 percent.

The NRDC assessment relies on certain assumptions about the difference between laboratory and real-world conditions, and analyses making different assumptions may calculate larger or smaller decreases. But critics and supporters of the move generally agree: Under this proposal, average real-world gas mileage would probably be significantly lower in 2025 than it would be if the Obama-era rules stayed in place.

Under the Trump administration’s proposal, fuel economy would continue to increase through 2020. But at that point the two rules diverge, with the Obama-era rule mandating continued increases in efficiency.

The difference between the two standards could amount to several hundred dollars a year in increased fuel expenditures for the typical motorist. Under the Obama rule, by 2025 the average vehicle would go through roughly 401 gallons of gasoline to drive a typical distance of 15,000 miles in one year. Under the Trump proposal, that would increase to about 507 gallons. Assuming, strictly for illustration purposes, a gasoline price of $3 per gallon, that would result in an increased annual fuel expenditure of about $318.

The Trump administration says that expenditure would be offset by reductions of several thousand dollars in the cost of new vehicles. It further argues that those reduced costs would allow more consumers to purchase newer, safer cars, which it says would save up to 1,000 lives annually. Many independent experts are skeptical of those claims.

“The law requires fuel economy standards in order to save fuel, and yet this proposal does just the exact opposite,” Shannon Baker-Branstetter, senior policy counsel for Consumers Union, the advocacy arm of Consumer Reports, said in a statement. “Thanks to emissions and efficiency rules, consumers have saved billions of dollars on fuel over the last five years, and pollution has dropped. Rather than capitalize on this progress and continue with plans to strengthen fuel efficiency and cut pollution, this move by regulators will ultimately leave consumers footing a higher bill.”