OK, a 6mpg increase could be due to almost anything. My tank mpg's on my Honda Civic are between 27 and 42 MPG. Did I do anything to get a 15mpg increase? No, the climate does, and my driving habits. If I put a gum wrapper on my side rearview mirrors, and the temp goes from 10°F before and I'm going into a 30mph headwind while maintaining 70mph on the highway, to 65°F the next day with a 15 mph tailwind going 55mph down state highways with hills. Do I post a 15mpg gain from the gumwrappers stuck to my mirrors? Heck no, not in here, your credibility goes to crap.
We know better. We know a 6mpg gain on a 25mpg car means a 24% improvement in mileage, which means a 48% improvement in aerodynamic efficiency. We know that most modern cars have fairly optimal front ends for aerodynamics, and that 70% of the aero gains that are possible come from improving the back portion of the vehicle.
So when we do the math, and see that the front 30% improvement potential, from a car already optimized fairly well, leading to a maybe 6% left on the table for improvement, and only half of this means an mpg increase, we see the 3% fuel efficiency potential being somehow 24%.
That's when the flag is thrown.
