FedEx runs such a massive operation—it uses more than 47,000 vehicles and nearly 700 aircraft to deliver about 4 million packages every day—that any systemic change it makes to cut down its carbon footprint can have major consequences. That’s why news that it’s using technology developed by a founder of Tesla Motors to make its trucks way more fuel efficient is so exciting, both for its investors and those who want the planet to breathe easy.

FedEx is working with Wrightspeed, the Silicon Valley-based company founded and run by Ian Wright, who helped create Tesla in 2003. Wright is still all about electric mobility, but his new company doesn’t make cars. It makes electric powertrains to be dropped into existing vehicles. And it’s sold 25 of them to FedEx for a pilot program.

The Wrightspeed conversion takes an existing conventional truck and replaces all the gas-powered parts that make it move. The engine, differential, and transmission are junked. An electric motor is matched to each drive wheel and a battery pack is thrown in to hold the electricity that powers them. The truck—now an electric vehicle—can be plugged in to charge the 39 kilowatt-hour battery, which holds enough juice to take the truck about 30 miles. Regenerative braking helps capture some extra power, but that's still totally lousy for a vehicle that spends all day driving. So Wrightspeed added an extra ingredient: a diesel-powered turbine to generate electricity while on the road.

Wrightspeed converts trucks into EVs with onboard diesel generators. Wrightspeed

The turbine is an internal combustion engine, burning diesel fuel. But instead of using the power it creates to pump pistons, it generates electricity. As long as there's diesel in the tank to power the turbine and create electricity, the truck can keep driving. When the tank is dry, it can be filled at a standard gas station (as long as it has a diesel pump). For the driver, very little changes except for a new instrument panel that shares data about battery charge and generation levels.

The system doesn’t eliminate the use of fossil fuels, but Wrightspeed says it can double the energy efficiency of FedEx’s fleet. That’s because the turbine system is especially well suited to delivery trucks. "If you think about a long-haul truck, cruising at 62 mph, that's the sweet spot for the engine" in terms of efficiency, says Wright. FedEx trucks rarely cruise on the highway. They spend their days in stop-and-go traffic, where conventional engines are not at their best, constantly and inefficiently moving through gears. Wrightspeed's turbine generator doesn’t change rev speeds, the way an engine does. It uses a consistent speed (or revolutions per minute, RPM) so it’s always operating at its peak efficiency point.

A more traditional plug-in electric hybrid like the Chevy Volt or new Mercedes S-Class can run solely on battery power for some distance before switching the combustion engine on, but the engine in those cars works harder when more horsepower is needed. In the Wrightspeed drivetrain, the gas-turbine engine generates a constant amount of energy with the engine running at a single speed. That enhances efficiency and reliability.

A year ago, FedEx purchased two Wrightspeed units as a trial. They were delivered in December last year. FedEx “just loaded it up, assigned a driver, and sent it out,” Wright says. On Christmas Eve in San Jose, CA one truck delivered packages for 14 hours straight. "Their expectations were very low," Wright says, but they started "using it like a regular trucks straightaway." The shipping company didn’t reply to a request for comment, but it’s clearly impressed by the new powertrains, since it’s placed an order for 25 more.

Wrightspeed is keeping mum on exactly what their retrofit kits cost, saying only the price is below $100,000. That’s about triple the cost of replacing an engine and transmission when those give out, but, Wright says, between the fuel savings and lower maintenance costs, its hybrid system pays for itself in just a few years.

The retrofitted trucks should be delivering packages by early next year, though Wright says that if FedEx or UPS decides to use its electric system on a big chunk of their fleet, "it's going to be stressful for our little company.” There are worse problems to have.