[basjoos]

A good (but not necessarily safe) way to measure your car's potential mpg improvement via Cd reduction mods is note how much of an improvement you get in mileage when drafting. When drafting, your engine rpm's are running at their usual levels, your rolling resistance is unchanged, and the only thing that has changed is your aero drag load. There have been some discussions in the past about how certain vehicle's gearing and engine loading don't allow them to to take full advantage of the reduction in engine loading resulting from a Cd reduction program, so this is one way you can test potential gains before going through the expense and effort of modifying your car to reduce its Cd. Ideally you should have an airspeed indicator in the nose of your car so you could graph the drop in airspeed as you get into a closer draft against the improvement in mileage. I'm not totally certain, but I'm guessing that the mileage achieved when the drafting airspeed has dropped by half would be the mileage achieved by your car when you have halved your Cd from its current value.

One thing I have noticed as I have dropped the Cd on my car is that drafting has less and less effect on my mileage and at a Cd of 0.17, it is not even worth bothering with even when a convenient draft presents itself. In some ways you could say I am drafting myself. Also the back of my car doesn't offer any mpg benefit to anybody trying to draft me, so my car is totally divorced from the drafting game.

The ability to benefit from drafting is a result of aerodynamic inefficiency both of the parts of the drafter and the drafted. An efficient aerodynamic design doesn't create the high and low pressure zones that can be somewhat canceled out (when you stick your high pressure nose in their low pressure wake) by drafting.