The U.S. Postal Service is adding seven electric delivery trucks to its fleet in Fresno and Stockton, California, a region in the state's central valley known for its poor air quality.

The 1.5-ton delivery trucks have been converted to electric power by Motiv Power, a Silicon Valley conversion company. 

USPS says it expects each truck to save between $4,000 and $6,000 a year on fuel costs and more than 37 tons of greenhouse gas emissions over the trucks' one-year trial. 

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The first Motiv Power mail truck began deliveries in Fresno on Tuesday. The remaining nine are scheduled to roll into service in Fresno and Stockton in coming weeks.

They're based on a Ford E-450 chassis and have a range of up to about 90 miles—plenty for a daily mail delivery route—and a top speed of 60 mph. It's likely the electric powertrain is much, much quieter than the diesels that are otherwise common on these trucks. 

California's electric mail trucks aren't the USPS's first foray into electric deliveries. In New York City, it uses 30 electric trucks, along with two 2-ton hybrid trucks on Long Island.

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The Postal Service is also in the midst of reviewing new vehicles to replace its aging fleet of more than 160,000 Grumman Long-Life Vehicle mail trucks that first went into service in 1987.

Among the four finalists are two fully electric models and one gas truck with a start-stop system. 

The Postal Service first used an electric mail truck to make collections in Buffalo, New York, in 1899.