*From the FKFS wind tunnel values,mpg might be predicted at 4-5.5%.
*If your car ran individually on the test track,on a calm morning,you might have experienced some laminar boundary layer conditions not present in the wind tunnel,skewing the aerodynamic drag downwards.
*In the United States,using the mass of fuel consumed would be considered a more accurate measure with respect to fuel consumption.
*If your 'Approach' angle is at least 16-degrees to the horizontal,then the bumper would have a chance to survive driveway ramps and such.(presently,some Volkswagen and Audi cars sold in the USA are losing some of their under-body components to ground strikes)
*The bumper is quite attractive and could evoke much emotional response in the market.