It all depends on the balance of drag forces on your car. Generally, there are three drag forces that use up the energy being applied to the wheels: rolling resistance, air drag, and changes in inertia (starting, stopping, etc.).

When you are plodding along at a steady 5mph, almost all of the drag on your car is rolling resistance. When you accelerate quickly from 0 to 15mph, the drag is split between inertia and rolling resistance. When driving at a steady 65mph, the majority of the drag is air resistance, followed by rolling resistance. Do you see how different scenarios have different balances of drag?

To determine the affect an aerodynamic chage will have, you have to determine what percentage of total drag is attributed to air resistance. Lets say it is 60% of total drag at the moment, the other 40% being rolling resistance (i.e. highway cruising). A 50% reduction in air drag will halve the amount of air resistance.

The vehicle now experiences

**70%** *(30% + 40%)* of the original total drag,

**43%** *(30% / 70%)* of which is now from air resistance. Theoretically, you should see a 30% improvement in mileage in those conditions. I hope the math is correct...

- LostCause