Any reduce in drag will result in better gas mileage. Drag slows down the car, so higher speeds can be obtained with decreased drag. Our longer diffuser body helps keep air swirl from behind the car kept to a minimum. Our added fins provide air channeling from front to back and they are bent to 90 degrees, so the air can't spill out of the sides. Our GTR Diffuser is constructed the same as all of our Elise/Exige/Evora and NSX diffusers, except this one took us a LOT longer to construct.
It's winter here in Montana, and there aren't any GTR's or wind-tunnels in the area at all. In the spring I'd like to make another test like this when I go to Seattle. For now, this video should demonstrate how the diffuser works, and we have a lot of customer testimonials. Diffusers are pretty helpful when it all comes down to it.
If you have any other questions, don't' hesitate to ask, PM or call! Happy to help!