The U.S. Postal Service plans to electrify a big chunk of its delivery fleet by 2028 in a move that it says will make it one of the biggest EV fleets in the country.

The switch from gas-powered trucks will mean a $9.6 billion investment in at least 66,000 EV delivery units, more than half of the 106,000 delivery vehicles USPS plans to buy in that time period. The service has more than 220,000 vehicles in its current delivery fleet.

Environmental groups had pressed the agency to stop using gas-powered trucks, including filing a lawsuit this past spring. USPS said in a statement that it will meanwhile continue to explore complete fleet electrification.

The new vehicles will have advanced safety technology and air conditioning, unlike the units they’ll replace, USPS said.

Funds for the EV purchases will come from the Inflation Reduction Act and from congressional funding.

Postmaster General Louis DeJoy said in the statement that, “We have a statutory requirement to deliver mail and packages to 163 million addresses six days per week and to cover our costs in doing so – that is our mission. As I have said in the past, if we can achieve those objectives in a more environmentally responsible way, we will do so.”

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