I suppose a long downhill is preferred to a steep downhill. But what about for uphill? For better mpg, would we want a short but steep hill or a very slight incline hill (for a longer distance), or something in between? Assuming all the hills have the same change of elevation and the same downhill past the crest.
I tend to prefer steep hills, especially if you are proactive about building a charge (sport mode). Too long of a descent and you will be engine braking. Trust me, coming through the Berkshires, it's LOUD, it smells, and there is absolutely no escape.
From an efficiency standpoint, the shallower longer climb should be more efficient than a steeper, shorter climb: Say I A) climb 1000 meters over 5 minutes vs B) 1000 meters over 10 minutes.
P= W x (h/t)
Watts = Weight(Newtons) x (height (meters) /time (seconds)).
For this example we will assume the Weight of the car to be 15,000 Newtons
The thing physics doesn't cover is ICE runtime; at a certain nexus it's better to run 100% load at time x, than 80% at time z.
There is definitely an ideal number for any grade/distance. My goal is to carry as much force into a climb as possible, and not to descend longer than I can regenerate energy.