I had mine installed last summer. It was louder undel exel and idle, but at cruising speed pretty good. I did get much better milage towing. Hand calc'd milage befor towing RV (10K lbs) I was getting 14-15 MPG. After install of 4040xl I got 17-18 MPG. I was very happy with the results. Worth the money I'd say. Towing in the mountains does get kinda loud after a while .. but with the money I save I can get some top line hearing aides !
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06 5.9 HO Mega 2500 Aero 4040xl, nano fiber drop in, totaly stock. Looking for minor mods to give better MPG's and not make a dent in the wallet !!