If you ignore the woeful thermal efficiency of internal combustion engines, and the effect of braking on economy you find that the great majority of losses for a car come through aerodynamic drag.
Drag is the force that's produced by the car the act of pushing air out of the way as it travels, and it increases as your speed increases. It's a squared relationship so if you double your speed, you multiply the drag by a factor of four.
Getting back to the point, the drag produced for a constant speed is calculated by multiplying the Coefficient of Drag (Cd) by the frontal area of the car (A). There's not a lot you can do about the Cd other than removing wing mirrors or spoilers, but you can reduce the A part by lowering the car. The reason that this works is because the air flow under a typical car is pretty disordered and 'draggy' so you can consider that area to be in the A part to a certain extent. If you reduce it by lowering the car then you can reduce A. This reduces drag, and increases economy.