The Tesla Cybertruck’s retractable tonneau cover is there for a reason. Well, multiple reasons, actually. Tesla says it helps keep valuable items safe, while owners might say that its sole purpose in life is to obscure the rear visibility when it’s closed (there’s a camera that shows what’s going on behind the vehicle, though.)

But there’s another reason, and that’s to improve efficiency, which ultimately leads to more driving range, and that’s always a good thing in an EV.

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The Cybertruck’s tonneau cover can make or break a long trip

Driving at high speeds constantly will make any EV’s range drop compared to its advertised figure. When it comes to the Tesla Cybertruck, though, there’s also the tonneau cover that needs to be considered. Running with it open or closed can make a pretty big difference during long highway drives, as Car and Driver found out.

Car and Driver ran multiple efficiency tests with a Cyberbeast-spec Cybertruck, which is the most powerful and expensive version of the angular electric pickup. It comes with a tri-motor setup that makes 845 horsepower, 35-inch all-terrain tires, and an estimated driving range of 320 miles, according to Tesla.

The testing was done on a 6.6-mile oval track at 75 miles per hour with the tonneau cover open, and then with the roll-top closed. C&D also removed the wheel covers and opened the tailgate to see if that would make any difference to the energy consumption of the stainless steel-covered pickup. Finally, all these tests were repeated at 65 mph.

Using the car’s consumption meter, the publication reset and recorded the cumulative consumption meter as the driver passed a fixed marker on the track.

At a constant speed of 75 mph, the Tesla Cybertruck Cyberbeast averaged 583 Watt-hours per mile (58.3 kWh/100 miles) with the tonneau closed, while the energy meter read an average of 642 Wh/mi (64.2 kWh/100 mi) with the tonneau open. That’s an almost 10% decrease in consumption, leading C&D to speculate that its 250-mile range test with the Cybertruck would have stopped after 225 miles if the tonneau was open.

Driving at 75 mph with both the tonneau and the tailgate open didn’t make a big difference, with the average consumption reading 640 Wh/mi (64 kWh/100 mi). Removing the wheel plastic covers amounted to a loss of just 1.7%, pushing the digital needle to show 593 Wh/mi (59.3 kWh/100 mi) instead of 583 Wh/mi (58.3 kWh/100 mi).

However, lowering the speed to 65 mph resulted in higher efficiency gains when driving with the tonneau closed. According to C&D’s testing, the average energy consumption was 493 Wh/mi (49.3 kWh/100 mi) with the tonneau closed, which is a 15.4% improvement compared to the 75 mph run. A quick round of napkin math reveals that the theoretical range would be 295 miles–45 miles more than what was achieved in the publication’s range test, but still below the advertised 320 miles.

With the tonneau open at 65 mph, the energy consumption increased to 542 Wh/mi (54.2 kWh/100 mi), which is 15.6% better than the 75 mph test with the roll-top retracted.

So, drive with the cover closed if you want to go as far as possible between charges in the Cybertruck.