The current fleet of delivery trucks used by the United States Postal Service have stood the test of time, but their high maintenance costs mean it's time to retire the iconic boxy vehicles.

The USPS began soliciting bids in 2016 and five finalists now remain in the running: they are Workhorse/Hackney, AM General, Karsan, Mahindra, and Oshkosh.

Two of the finalists offer electrified options—Workhorse and Mahindra—and environmental groups are urging the USPS to choose an electric future over one powered by fossil fuels.

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In fact, 13 groups penned a letter to the USPS last week in an attempt to persuade the government entity to choose an electrified delivery truck.

One signatory, the Sierra Club, not only highlighted the initiative but urged its members to tweet at @USPS to encourage the postal service to go electric.

If the USPS does choose an electric delivery truck, it would become the largest electric-car fleet in the world.

U.S. Postal Service Grumman

U.S. Postal Service Grumman

The Workhorse/Hackney proposal is based on the company's W-15 range-extended electric pickup truck.

It boasts an 80-mile electric range before small gasoline engine from BMW kicks in as a range extender.

The USPS requires the trucks can hold 1,500 pounds of mail, but the proposed Workhorse delivery truck can hold up to 2,200 pounds, which may help its chances.

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Indian automaker Mahindra's bid features a 2.5-liter inline-4 engine with a mild-hybrid powertrain from General Motors.

That makes it greener than the current mail trucks, although not nearly as efficient as Workhorse's proposal.

The familiar USPS long-life vehicle (LLV) used today was designed and put into operation in the 1980s.

Workhorse W-15 plug-in truck

Workhorse W-15 plug-in truck

More often than not, operators says, they do not meet their estimated 16-mpg fuel-economy ratings.

Advocates called for the switch to electrified delivery trucks both to curb emissions and to improve mail carriers' health.

"Because the Postal Service delivers to every address in the nation, they have a unique responsibility to consider the impacts of their trucks on its employees, neighborhood residents, and taxpayers," Sierra Club Clean Transportation for All Director Gina Coplon-Newfield, said.

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"From the public health benefits to cost reductions, electric delivery trucks make sound sense for the USPS," she added.

In most areas of the country, plugging in an electric truck to recharge its battery pack overnight also cuts per-mile cost significantly over a diesel equivalent.

The USPS will award its new contract, with a total value as high as $6.3 billion, in early 2018.


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