I was wonder if anyone here has ever put a Flowmaster muffler on their TV. I just put one on and I noticed an increase in the horsepower when not towing and no or little change when towing. I have a 2003 Expedition with a 5.4L 4X4 with a complete towpackage. I noticed that before I changed the muffler the car would change into Overdrive on very flat highways (I always travel the speed limit - 55 mph in CA when towing a trailer). After I changed over the car very seldom changes into OD. Thus my mpg is lower by 1 mpg. Can anyone offer any explanation for this?