To summarize my thoughts it depends on the length/grade of the corresponding inclines and declines. Assume my car gets 60mpg on a flat surface.
If I drive up a 6% grade for three miles (900 foot elevation gain) and then proceed down a 6% grade back to the original elevation it's a net loss because I cannot properly utilize all that free downhill energy since I would be dangerously exceeding the speed limit and experiencing massive wind resistance at the speeds generated by the downgrade in neutral. Plus since my engine still runs going downhill I'm still using some fuel.
However if that three mile 6% uphill gives me nine miles of 2% downgrade I believe I will come out ahead. Sure I may only achieve 35mpg for three miles uphill, but the 80mpg on the nine mile downhill will make this trip even or even a small net gain possibly.
Some real world examples:
The huge descent on I-80 into Salt Lake City is a huge waste of downhill energy since I have to stay in 5th and BRAKE to maintain a safe speed (under 85mph) on that incredible downgrade.
The ascent to the top of the "Grapevine" north of Los Angeles on I-5 is very steep and quick, but the resulting downgrade offers twentysome miles of very mileage-friendly downgrades of less severity. A great example of corresponding grades that are GOOD for fuel economy.
All depends on the length and severity of the corresponding grades IMO. I use this to my advantage, since I CRAWL up steep hills and milk as much free energy as possible going downhill in neutral.
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