What was funny on my car is that the gt500 rear fin reduced my mpg by 0.4 down the highway using my old test trip (test trip mpg- 26.4, -1.5%). And it had lots of front undertray around the motor. I had to pull the one under the engine off because it made the front end "flutter" so much from the turbulence under the car it caused. With the tray installed the hood moved at 75mph the same amount it did at 120mph without it! Helped with NVH cruising down the highway as well. Overall dropping the car 1.5-2" and removing the under tray I never noticed a change in mileage from either.
https://drive.google.com/file/d/0B1A...ew?usp=sharing