There must be something wrong about it, but I can't put a finger on it.
- With a land based vehicle to stay at one place you don't need any engine power, just hold the brake, which means zero energy
- You can't stay at one place with engine power. It would need zero rpm with any gear ratio, so the engine would not turn at all, and would not use any fuel. But it also wouldn't work because gravity would be able to turn the engine backwards so the vehicle would roll down.
- IRL there's a way to stay at one place with engine power: to rev it and let the clutch slip exactly that much that the remaining torque is enough to balance the force of gravity. But in that case the engine would do no useful work (every single bit of its movement would be converted to heat in the engine and the clutch) thus its efficiency would be zero.
So what's wrong here?
Anyway, I usually accelerate uphills to get longer glides on the other side (given that I don't know of a sharp turn or stop sign at the bottom) - I ride a motorcycle and those are light and have bad aerodynamics so on slight downhills they can't even hold a decent speed let alone accelerating from there.