>>13712409The energy wasted comes from the kinetic energy of the vehicle, not from fuel being burned by the engine.
Almost any old and all modern EFI-equipped cars have cut-off on coast, meaning your injectors don't fire at all as long as the accelerator pedal is not pressed and the engine is above idle speed. This feature has been invented mainly for two reasons: 1- it saves fuel and 2- it allows for more engine braking, which in turns allows for less brake wear.
So, assuming the decline is steep enough that the car won't need to accelerate again down the road because it slowed down too much, then the choice of gear engaged has absolutely no impact on fuel consumption, only on brake wear.
When going down, you either scrub that kinetic energy by turning it into heat via the brakes or via engine braking.
If you keep the engine idling, you are burning the fuel needed to maintain that engine speed.
If you keep your engine above idle with a gear engaged and no accelerator applied, then you burn no fuel because the engine is turned by the wheels alone and doesn't need to be "helped" in maintaining a certain speed.
Your logic on hybrids is correct and using regenerative braking will help you save fuel once you have to accelerate again, but if we are talking about cars with no hybrids and no carburettors, then coasting burns no fuel at all, regardless of the gear engaged.
So, going downhill with a low gear (assuming no need to accelerate until the decline is finished) = more engine braking, less brake braking, no fuel used
Going downhill with an high gear (same assumptions as above) = less engine braking, more brake braking, no fuel used
Going downhill in neutral woth idling engine = no engine braking at all, 100% brake braking, some fuel used to keep the engine at idle speed.
