ICEs are not terribly efficient in most of their rev ranges, especially toward the lower end of the range where they are not near peak torque or peak horse power.
If you are cruising on a level grade you will achieve peak fuel economy by selecting the highest gear that can still maintain your current speed, it requires very little horsepower to maintain a constant velocity on a level grade so you are able to run your engine in its lowest rev range resulting in the best fuel to power ratio.
That is no longer true once you begin to require more horsepower/torque, either because you are accelerating or climbing a hill. If you shift too early, and thus drop your available horsepower/torque, you will burn more fuel getting the engine to climb back through its inefficient range than you would have if you had held the lower gear and shifted later. If you have any experience driving a car with a manual transmission you have probably noticed this effect. Recall times when you may have accidentally shifted to a higher gear than you intended, grabbing 5th instead of 3rd, or stayed in a higher gear while rounding a curve. It required a lot more throttle to achieve the same rate of acceleration in 5th gear – likely floored – than it would have if you had shifted to 3rd.
In general, it is more efficient to keep an engine in its power band while accelerating, and to allow the revs to drop as far as possible while cruising at a constant speed.
You can find additional research by Googling Power Bands, Internal Combustion Engine Efficiency, and by comparing the efficiencies of electric motors to the internal combustion egine.