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What you say is true, but I'm not increasing my speed. Only my RPM. I have cruise control on. Here's what's really happening: as I get to a hill in 4th gear, there is a much higher load on the engine while cruise tries to keep up (throttle position increases quite a bit). When I downshift into 3rd, engine RPM increases and MAP decreases (if I go from 4th to 3rd when the MAP is initially 24, then it will drop to 20). This results in the engine being in a more powerful RPM range, and the cruise will dial back the throttle. So, what's really occurring is that the throttle needed to maintain speed up a hill is lower when the engine is at a slightly higher RPM. In doing more testing, I've found that MAP is a good way to keep track of what gear to be in, but at the end of the day, less throttle is more MPG (regardless of engine RPM). In other words, if I maintain throttle position at any given speed and shift gears, MPG barely changes.
I still assert, though, that 20inHg is a sweet spot (for my engine) as far as MPG is concerned. My next testing, which will be conducted someday in the future, is to use a CVT and constantly hold the MAP at certain values and take measurements at each.
Basically, what I've been doing is finding the balance between available engine power and demand. Having the engine at a place where power is not available (low RPM) when there is a high demand for power will result in overloading of the engine and decreased MPG. Inversely, with the engine at a place where available power is much higher than demand will decrease MPG considerably as well.
Last thing I'm going to touch on here is that my MPG readings are instantaneous, so miles traveled doesn't make a difference. It's MPH at that instant, which I am keeping at a constant value.
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