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The two major causes of high gas consumption at cruising speed are wind drag and pumping losses. If an engine is perfectly sized so that it requires maximum HP to cruise at some speed, the throttle will be wide open and pumping losses will be minimized. However, no one would want such an underpowered car. There are two ways to reduce pumping losses, the first being used by some cars is to shut off some cylinders. This requires a wider throttle opening to produce the required HP, reducing pumping losses. The other way, which is superior, is to have additional ratios in the transmission to reduce RPM's to a low level. In order to maintain the required HP the throttle has to be open more. Many cars now have 6-speed or even 7-speed transmissions for this purpose. So, by having variable valve timing, which allows the engine have lots of torque over a wide RPM range, an engine can comfortably operate at a very low RPM, where the maximum HP is minimized, and the throttle is open quite far. So despite your high school physics expertise, you really can have lots of power and good gas mileage. We'll talk about engine friction losses some other time.
