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Ah, theory vs reality. Sometimes the same, sometimes not. I have a little puzzle. An airplane in Alaska is picking up mail from a small remote village and needs to fly from the city to the village and back in the same day. The airplane cruises at 150 MPH and burns 10 gallons per hour. The next city is 300 miles away. If there was no wind the trip would be two hours each way and burn twenty gallons each way. For a total flight time of four hours and 30 gallons burned for an average 15 MPG. If for example there was this day a fifty MPH headwind that remained steady the whole day, then what would the stats be? It's similar to our hill if we try to maintain some speed limit. Try and figure it out. It's not hard and the answer is revealing.
Back to our car going up and down a hill. The internal engine drag is proportional to the square of the RPM (but there is a lower efficient limit) and the computer will enrich the mixture above a certain throttle setting (and at idle to run smoothe) to help cool the engine internally. So it isn't a simple intuitive call as far as the uphill portion of the hill is concerned. You could plot a graph if you knew the exact data and exact changing conditions. I think however to get the max MPG up hill you will need to know in general the best operating range of your engine as far as throttle position versus RPM versus Speed Versus current gear ratio. Going down hill you will probably peak the unit unless the grade is gradual. IMHO that boils down to using general principles of high mileage and learning to peak your instantaneous MPG via a ScanGage, etc.
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