Medium- and heavy-duty fleet trucks would have to meet stricter fuel efficiency standards under a proposal by federal environmental and highway regulators, part of the Obama administration’s effort to reduce climate-warming pollution across the economy.

The standards proposed on June 19 would cut greenhouse gas emissions by about 1 billion metric tons over the life of the nation’s fleet, agencies said.

That’s roughly equivalent to the pollution linked to electricity use by all U.S. households for one year. The standards — covering tractor trailers and the largest vans and pickup trucks — would begin in the 2021 model year and be applied through model year 2027.

For trailers, the standards would take effect in the 2018 model year.

The transportation sector is the second-largest contributor to the U.S. carbon footprint, after the utility industry, according to the EPA. Medium- and heavy-duty trucks emit about 20 percent of the sector’s carbon pollution while accounting for just 5 percent of the vehicles on the road.