A lot of what has been said is true, of course it all has to do with gearing and how you drive. Don't buy a 4:10 dually and think you are going to get 19!! The best I have gotten was 17.2 hwy, I was really nursing it and kept it under 2K. It wasn't fun. I just got back from towing my 10K TT for a full tank, I averaged 10.2 mpg and I was really pushing it hard the last 100 miles with the CC set on 73 mph going through some rolling hills. You don't need a 4" exhaust you just need a free flowing exhaust, this means get rid of the factory muffler, and replace with a BTM or straight pipe it, cost is about the same. Get your gauges first.
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