To return to airplanes and engines of a slightly smaller scale, the power required, and hence the fuel consumption, of airplanes, over the range of normal cruising speeds, is approximately proportional to the cube of the speed. I don't know by what divine dispensation we began thinking of 75, 65 and 55 percent of power as the standard power settings for cruise, but as a matter of convenience we can stick with them for the sake of this discussion. If 75 percent of rated power is taken as 100 percent of cruising power, 65 percent becomes 87 percent (because 65 is 87 percent of 75), and 55 percent becomes 73 percent. Other things, like altitude and mixture setting, being equal, the corresponding speeds will be, respectively, around 95 percent and 90 percent of the maximum cruising speed (because .95 is the cube root of .87, and .90 is the cube root of .73 -- any errors you might spot are due to rounding). I know this sounds a bit cut-and-dried, and different airplanes will deviate a little from the rule because of different engine or propeller characteristics; but if you check a few performance charts you will find it to be quite close to the truth.