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Every hill is an opportunity. You can store energy in your vehicles increase in altitude, then use that same energy to coast downhill. Some of my best fuel mileage was driving in hills, even mountains as long as you can coast without having to use engine braking or friction brakes.
Think of it as the perfect hill scenario.
Climbing at best BSFC in your highest gear is most efficient use of the engine. Coasting down the other side of the hill allows you to use engine off or engine idling. Even with the engine idling my Insight would get 150 MPG coasting at only 16 MPH. The Altima would get 60 MPG at the same 16 MPH speed.
If you are coasting at 64 MPH, in the insight that was 600 MPG with the engine idling, and 240 MPG in the Altima. Both were automatic transmissions so I just coasted in neutral with the engine running so the transmission pump was still providing pressure for lubrication in the transmission.
In the ideal hill scenario, where you could climb at best BSFC then coast, while maintaining the same speed, it is actually more efficient than regular flat terrain P&G since the constant speed means less total aero drag than the average of peak speed and the lower speed at the end of your glide, due to the exponential increase in aero drag as speed increases.
regards
Mech
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