[RedDevil]

By driving up the hill you add a lot of potential energy by moving the mass of your car further away from the earths core. It's not lost; just converted from fuel to height.
Thats the way with physics; the law of gravity is strictly implied and perpetraters will be brought down infallably

A typical car engine will perform more efficiently under a heavy load than under a very light load.
Going down it uses no fuel at all. With the right incline you could even be better off than driving on flat ground; a natural P&G opportunity.
As your engine already runs efficiently up that hill there is no real room for improvement, except some weight shedding. It ain't broken, so don't fix it.

Wouldn't use EOC going down a mountain road; that's when I would not like to suddenly lose PS or brake support. The economy going downhill will always be impressive, even if the engine does not shut off fuel intake completely.