A lot of pickups are boxy. The Cybertruck isn't exactly svelte, but it apparently has much better aero and mileage. I've been wondering for awhile: Would a sloped bed cover going from the top of the cab to the gate increase the mileage of any pickup truck by some whopping amount? It seems like the aero would go from "ginormously" bad, to just "meh," which could still be a lot.
With the right kind of aerodynamics you can get a sort of bubble of air of roughly the right shape. Having a hard surface of that shape (wouldn't be exactly sloped as much as a bubble shape without discontinuities on the trailing edge) would be optimal, but you get 'ginormously bad' by having a sharp discontinuity without any turbulence generation.
That can be either the little nubs of high performance cars, or something like Airtabs that are meant for big-rig trucks. And either way you don't get to increase the mileage by a whopping amount as there are still probably shape issues on the leading edge of the vehicle, plus sheer surface area. To do amazing streamlining the whole thing has to be a bubble, including the front edge of the vehicle.
The tonneau cover industry likes to quote a senior-thesis type paper from university engineering students that even one of those improves aerodynamics by 10%.
And modern trucks aren't as bad as they used to be.
Modern trucks are better than they used to be, but they seem to give most of the efficiency back in terms of larger towing/payload capacity. It's absolutely bonkers how capable a "half ton pickup" can be equipped compared to 20 years ago.
