Sorry Cardinal,
back to physics class, please...
fuel consumption is dependant on throttle position - on that count you are right - and RPM, unfortunately.
As MetroMPG once said, and I can support this fully: I have yet to see a car which uses less fuel when cruising at higher RPM. Internal engine friction and pumping losses wil see to that! Apart from uphill situations or when accelerating, less RPM always means less fuel consumption as long as the engine can still hold the speed without flooring the pedal. So early upshifting (in my case, my diesel never sees more than 1600 rpm in first to fifth gear, sixth gear is engaged from 70 km/h on) and long last gears are absolutely essential for economic driving.
Furthermore, double fuel consumption does not mean double speed (and therefore not double traveled distance per fuel unit) unless you are traveling in vacuum or talking about ridiculously low speeds. Air resistance rises as a quadratic function, so going from 50 mph to 100 mph, you will more or less quadruple your fuel consumption while only doubling your traveled distance per time.
so long,
tinduck