Just put it on weeekend before last, did nothing but muffler change. Pretty quite, not much louder than stock but a nice sound and you can hear the difference when you put your foot down. Made a trip to gas station to fill up and could really feel a difference. This past weekend was the first real test. Pulled a 3500 lb B-B-Q pit on tandem trailer 160 miles in stop and go traffic. I commented to my wife that I could feel an improvement in power after the 4th stoplight and her comment was, "It feels a little sluggish to me." When I reminded her we were pulling the trailer she was surprised, she forgot we were towing, then she was impressed(beleive me, she is not real hot on modifications to a new truck). Got back yesterday afternoon and filled the tank, didn't fudge, I topped it off,12.1 gal., comes out to 13.22 MPG. Not bad as this is about what I got in stop and go with no load.
Next test, road trip. The best I've done to date is 19 mpg. I've got Air Raid on order (wife doesn't know about this yet) and will probably put it on before I get to try trip with just muffler change. My truck is primarily weekend only, I've had it a year and just turned 7,000 miles. First new truck I've ever owned, just never could justify buying a new one as little as I use it. So glad I did and this message board makes it so much more fun. C-Ya....

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98 Nascar Edition, Superchip, K&N with airbox mod., Flowmaster 70 series, bedliner, in bed tool box, Profile bug deflector and window vents, Class III receiver hitch.