WASHINGTON — President Joe Biden, a self-described “car guy,” often promises to lead by example on climate change by moving swiftly to convert the sprawling U.S. government fleet to zero-emission electric vehicles. But efforts to eliminate gas-powered vehicles from the fleet have lagged.

Biden last year directed the U.S. government to purchase only American-made, zero-emission passenger cars by 2027 and electric versions of other vehicles by 2035.

“We’re going to harness the purchasing power of the federal government to buy clean, zero-emission vehicles,” the president said soon after his January 2021 inauguration. He has since used photo ops — taking a spin in Ford Motor Co.’s electric F-150 pickup truck, or driving GM’s Cadillac Lyriq electric SUV at the Detroit auto show — to promote their potential. Cabinet officials have hawked a first set of Ford Mustang Mach-E SUVs in use at the departments of Energy and Transportation.

The White House frequently describes the 2027 timeline as on track. But the General Services Administration, the agency that purchases two-thirds of the 656,000-vehicle federal fleet, says there are no guarantees.

Then there is the U.S. Postal Service, which owns the remaining one-third of the federal fleet. After initially balking and facing lawsuits, the agency now says that half of its initial purchase of 50,000 next-generation vehicles will be powered by electricity. The first set of postal vehicles will hit delivery routes late next year.

Climate advocates say that agency can do even better.

“USPS should now go all-electric or virtually all electric with its new vehicles,” said Luke Tonachel, senior director of clean vehicles and buildings at the Natural Resources Defense Council, citing an additional $3 billion in federal spending targeted for the postal fleet under the landmark climate law Biden signed last month.

About 30% of U.S. greenhouse gas emissions come from the transportation sector, making it the single largest source of planet-warming emissions in the country.

Electrification of the federal fleet is a “cornerstone” of Biden’s efforts to decarbonize the federal government, said Andrew Mayock, chief federal sustainability officer for the White House.

“The future is electric, and the federal government has built a strong foundation … that’s going to deliver on this journey we’re on over the next decade,” he said in an interview.

Excluding the Postal Service, about 13% of new light-duty vehicles purchased across the government this year, or about 3,550, were “zero emissions,” according to administration figures provided to The Associated Press. The government defines zero emissions as either electric or plug-in hybrid, which technically has a gas-burning engine. That compares with just under 2% in the 2021 budget year and less than 1% in 2020.