The photographers say they heard the hum of the electrics, followed by the noise of an engine kicking on around 25-30 mph on multiple occasions while tailing the truck seen here. Our previous story speculated that Toyota could use a hybridized version of the 3.5-liter twin-turbo V6 we saw make its debut in the Lexus LS500. Another possible powertrain option could also be borrowed from the LS500h. That one uses a naturally aspirated V6 and an electric motor for a total system output of 354 horsepower and 369 pound-feet of torque. If it's the former, we'll expect significantly more power, with the powertrain likely being considered for a flagship version of the truck. Our guess would put a hybridized version of the twin-turbo V6 at about 450-500 horsepower.
This particular tester doesn't look like a "flagship" truck with the small, steel wheels, but who knows what Toyota is up to with its test vehicle. Physical details of the Tundra mule we see here are similar to the previous set of photos we got. Toyota is still trying extra hard to make sure we can't see what's going on with the rear suspension, implementing all sorts of brushes and blockers. Everything rear of the cab is kept tightly under wraps, and the front end gets the same treatment. None of the lights appear to be production units at this point, but they're covered up nonetheless.
A previous report told us that the new Tundra and Tacoma would share a platform internally called F1. We expect the first truck on this platform to be revealed in 2020, so that tells us we'll see the new Tundra next year sometime — the Tacoma likely still has a bit of a wait until its next overhaul. Hybridization, with still unknown amounts of power, will likely be along for the ride when the truck is finally shown.