I know at this point it isn't very accurate but, I was greatly suprised with a increase of fuel milage.
I installed John Kennedy's 3.5" exhaust system and have driven the first complete tank since it was filled.
Most of my driving is mixed and don't spend a lot of time on the highway except on vacations.
Normally it has been averaging around 14 mpg and this one tank averaged 16 mpg.