Ford’s F-150 is going hybrid, but could it go diesel too?

Ford CEO Mark Fields says the company will launch a hybrid pickup truck before the end of the decade, but that may not be the only efficiency-focused powertrain Ford is planning. Rumor has it that there’s a diesel F-150 prototype puttering around the carmaker’s Dearborn, Michigan, hometown.

A somewhat camouflaged F-150 sporting a Venturi exhaust tip indicates Ford is testing a diesel engine for its bestselling pickup truck, according to Autoblog. While a hybrid would be quite novel, a diesel F-150 could give Ford a powerful weapon in the truck fuel-economy wars.

Ford just certified a 3.2-liter five-cylinder diesel engine for the Transit van, but the engine it’s eyeing for the F-150 is reportedly a 3.0-liter V6. Codenamed “Lion,” it’s the same engine used in the recently-launched Land Rover Range Rover and Range Rover Sport Td6 models. Land Rover used to be owned by Ford.

The V6 produces 254 horsepower and 440 pound-feet of torque in the Land Rover models. If it gets installed under the F-150’s aluminum hood, it will reportedly be mated to a new 10-speed automatic transmission. This transmission will debut in the 2017 F-150 Raptor performance truck, but is expected to find its way into other F-150 models, and maybe even the Mustang.

With the current-generation F-150, Ford went to great lengths to improve fuel efficiency. It gave the truck a lighter aluminum body, and added a new 2.7-liter turbocharged EcoBoost gasoline V6. But the Ram 1500 EcoDiesel and its 3.0-liter diesel V6 still beat the F-150 in fuel economy. The diesel engine and 10-speed automatic combination might be what Ford needs to finally beat Ram.

Diesel engines are becoming more popular in trucks as manufacturers look to increase fuel economy without upsetting notoriously picky truck customers. The redesigned 2016 Nissan Titan XD boasts a 5.0-liter diesel V8, and General Motors offers a 2.8-liter four-cylinder diesel in its pair of mid-size trucks, the Chevrolet Colorado and GMC Canyon.