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If you have to do a total system you would have a minimum and maximum speed. if not i would supose you would know you are going to slow. But that is the whole purpose IS to slow up going up hill. Do you have a scangauge. if not you wouldnt understand what maintaining a mpg setting details.
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Yes, I do have a Scanguage, and in PA, we also have some fairly steep grades that are miles long. While I can easily hit 38 MPG on the entire trip, it is absolutely impossible to maintain even 30 MPG in any shape or form for a certain 1.5 mile stretch. Trust me, if you try to maintain cruising MPGs up extended hills, you will have to slow down to a stop. Your way only works if your car's momentum is enough to put you over that hill.
With hills like these, I stay in 5th and give it about 80% throttle to maintain speed. A greater percentage of the fuel I burn gets turned into motion at higher loads, so I take advantage of it.
Also realize that if I were to maintain 38 MPG instead of the infinite MPG I get going down that hill by laying off the throttle, I'd soon be at 110 MPH.
Maintaining a certain MPG is pointless. You'll be able to average even higher MPGs by using moderate throttle up hills and zero fuel down hills. There's nothing wrong with gassing it up hills if you know you'll be able to coast down the other side without riding the brakes. Take a look at a BSFC map sometime.