After reading through this tread yesterday and looked at the valve on my wife's X3. I noticed that it was open so I watched the valve as she cranked up the car. It stayed open for about 2 seconds after the car was cranked and then it closed. I could tell a difference in the sound when it closed. Also I always thought that a free flowing exhaust (less back pressure) would produce more HP and torque. That's usually where most of your HP gains come from when you put a Flow-master or Borla exhaust system on. I would think that having the valve open would produce a little more power which might help a little on gas mileage. As for 5 mpg.... I would be very impressed but I would be more inclined to think you aren't as heavy on the throttle because of the increased sound at lower speeds. Just my 2 cents... if I am wrong let me know.