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As the load on your engine increases the efficiency increases, as well as the fuel consumption.
If 20 HP takes 1 unit of fuel, 50 HP takes 1.5 units of fuel, so the extra 30 HP cost you only half again as much fuel as the first 20. Therefore the cost per HP goes from 10 HP per half unit to 16.66 HP per half unit.
Its because you are getting higher effective compression with less throttle restriction, and your "lever" is stronger.
I would actually let the speed drop slightly uphill then recover it downhill if the grade is steep enough, but I think that is not your case.
In my Insight I accelerate very gradually uphill, and the mileage stays above 55 MPG. Then I semi coast downhill and the mileage jumps to 125+ MPG.
These are very slight grade hills, probably 1% grade, maybe even less.
I have seen readings of 95 MPG on that section of road, no traffic lights, and very little traffic since the road runs parallel to the Interstate.
I doubt you will see much gain if you have to pulse to 70 MPH. If you are trying to average over 50 MPH, the peak speeds you must reach can double the aero drag of your average speed and negate and increase in BSFC in your engine.
regards
Mech
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