[aerohead]

Primarily,what kind of driving will you be doing? Drag reductions will show very low returns in an urban setting.
If, however,you'll do a significant amount of constant-speed highway cruising,then the aero mods will definitely be in their element.
Also,you need an accurate mpg baseline created at the highway velocity you'll be driving or else we'll never be able to predict a thing,nor have anything to compare to after mods are in place.
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Some parametrics which are of interest:
*Coefficient of aerodynamic drag (if published)
*Projected frontal area (if published)
*Length
*Width
*Height
*Ground clearance
*Highest point on roof when there's 300-pounds of weight in the car
*Widest point of body when looking down from above.
*Degree of underbody 'roughness' compared to a smooth belly
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Any meaningful drag reduction will come from the aft-body of the car.Thule makes a 1-inch receiver hitch for bicycle carriers which could form the anchoring point of a removable tail section,snugged by tension straps.
If you're looking to really cut drag you'll need to think in this direction.