Aerodynamic effects take over at between about 30 to 35mph. So below that weight of the car is the predominant factor.
2 factors influence the aerodynamics, frontal area and drag coefficient.
Drag inreases with the square of speed. 70 to 80 is generally accepted to be associated with a 15 to 20% decrease in consumption. If you have a high frontal area (mini MPV / SUV) it will be worse than a slippy saloon.
But this is all steady state. what influences consumption more is accelerating. How you drive is most important. Commuting at 80 on an empty motorway will produce better figures that 80 on busy commuter time motorway.
My best consumption figures are produced on cruise control. Using this it is simply staggering how much everyone else is varying their speed.