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Maintaining a relatively constant speed up a very slight grade puts the engine under a higher load which increases efficiency. This will either give you better mileage or a lower decrease in mileage. I used to use this in my 02 Insight CVT (not lean burn) on very gradual grades east of my home to get much better MPG. I would very gradually increase my speed, with a very slight reduction in MPG. Then on the downgrades my mileage would skyrocket.
This has also been noticed on Prii and some other cars. By storing your inertial energy in climbing a very slight grade you can coast down the backside of that grade with much higher mileage (100% or better than your average).
The increase in BSFC when climbing a very slight uphill grade means you are getting more energy for the same amount of fuel used. It is similar to P&G but the grade changes can (in some instances) allow you to do this while maintaining a much more consistent speed. In the ideal scenario you could go a constant speed with better mileage overall.
A slightly higher load climbing a very slight (1-2%) grade allowed you to get higher (than your average) mileage, but going down the same grade on the other side of the hill (assuming there is the "other" side), is a golden opportunity to coast (engine on or off) and see a dramatic increase in mileage.
regards
Mech
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