[PaleMelanesian]

In one sense, that is right. In another sense...

You used fuel going up the hill. You gain that back on the downhill, MINUS the drag being overcome, which includes aero and engine braking.

I would try to crest the hill even slower, so the trip down the slope doesn't approach terminal speed. At the very least, I'd try to minimize the time spent near terminal. To the extreme, if you crest at 2mph, you've used the absolute minimum fuel required to reach the peak, and gravity will take you all the way down for free. Obviously that's not a usable solution, but the concept can be applied in moderation.