The fuel usage for a vehicle is not linear (straight). There is a threshold to maintain operation (600-1000 rpm depending on vehicle) that just keeps the ICE and powertrain running. This is "wasted" rpms/fuel. Beyond that, fuel usage goes up linearly with rpm. However, POWER (not rpm) is what supplies the energy to overcome friction and aerodynamic drag. However, the available Power to the wheels does not go up linearly. It varies according to the "Torque Curve" because of the nature of an ICE. Thus, fuel usage and speed are not proportional. Also, aero drag is a function of the SQUARE of the Speed, so it takes more power at higher speed to maintain cruise. Thus, every vehicle has a series of overlapping slightly misshaped bell curves that determine the most efficient speed at each gear. The most efficient point is each gear is the point with the lowest RPM/Speed qoutient for each gear. The most efficient overall speed is the speed with the lowest RPM/Speed quotient period. Generally one of your top 2 gears. Going back to my truck, it turns at 1600 rpm at 55 and 1800 at 65, which means it is 5% more efficient per MILE. At 75, it turns 2300, which is 5% less efficient than 55 and 6% or 7% less efficient than 65. At 45, it turns 1325, which is less efficient than 55 but saves the acceleration gas. Hope that helps.