For many, the electric transport revolution can’t come soon enough. Battery electric vehicles could make up 57 percent of global passenger car sales by 2040. But researchers at MIT and the Ford Motor Company show that, in the near term at least, lightweight gas-powered vehicles could make more environmental sense in some parts of the U.S.

Electric vehicles reduce greenhouse gas emissions for most of the nation, they report in the journal Environmental Science & Technology. The impact is especially larger on the two coasts and in the south. But in chunks of the Midwest, lightweight gasoline-powered vehicles could have a greater impact on emissions.

The transportation sector is one of the largest sources of greenhouse gas pollution. Plus, tailpipe emissions lead to smog and tiny particle pollution, which can harm health, especially of children and older adults. Replacing gasoline-powered cars with electric ones could reduce these emissions. But skeptics point out the dubious greenness of EVs. Manufacturing batteries can have an environmental impact, they argue, as can the source of electricity used to charge the vehicles.

The new study considers a variety of factors that contribute to the environmental impact of EVs at a county-by-county level across the U.S. These factors include differences in regional climate, the different power-generation sources, and the average number of miles driven every year.

The source of the power going to the grid played the biggest role in the emission burden of a vehicle over its lifetime. The grid is relatively cleaner on the coasts with the use of natural gas power plants and renewables. But the Midwest still relies considerably on coal plants.

Not too many studies have taken into account ambient temperatures and regional differences in driving patterns, the researchers point out. But these factors can have important effects on emissions. Heating in very cold climates, for instance, can significantly drag down battery efficiency. Meanwhile, urban versus rural driving conditions can affect fuel efficiency as well, because EVs are more beneficial in stop-and-go traffic.

The team used these factors to compare emissions from standard midsize gasoline cars, lightweight versions of these cars, and electric cars. Lightweight cars are those with structures made mostly from aluminum or specialized lightweight steel. Data for standard cars came from Ford’s vehicle-performance data on around 30,000 cars. The researchers used standard modeling techniques to calculate the performance of the other vehicles.

In some locations in the Midwest, they found, lightweight gasoline cars would actually result in fewer emissions than buying a comparable electric car. This advantage in emissions-reduction was the largest in parts of Wisconsin and Michigan. “Some of the areas with more carbon-heavy grids also happen to be colder, and somewhat more rural,” said MIT’s Randolph Kirchain who led the work. “All three of those things can tilt emissions in a negative way for electric vehicles.”