[KamperBob]

I agree an air dam could hurt MPG. In free flight a streamline body has nominal low Cd. In ground proximity its Cd goes up substantially. If a chin spoiler adds frontal area then unless it somehow reduces Cd enough to compensate then it seems reasonable to expect CdA to increase. To my mind just cleaning up under belly seems more likely to improve Cd and Cda. Just my 2c...