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People confuse 'speed' with 'energy'.
Unless the hill is so steep that you have to drop back gears or go into fuel enrichment, gaining momentum has the same cost on the flat, up hill and down hill. What goes up, must come down, essentially.
Ideally, you want to crest the hill at such a speed that you end up at the speed limit (or your target speed) at the bottom.
Only accelerate on descents if the next hill you face requires that extra momentum. Otherwise, climb slow and steady, in top gear at BSFC (80% load).
*All assuming you have a NA, throttled spark ignition engine.
I've run a test on hills and indeed you get better economy with hills than without. I have a great hill that I use regularly. It's a steep climb, followed by a shallow decent. BSFC up, EOC down. Crazy mileage.
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