RE: Cummins 6.7 mpg
24ssm I think you have your city/highway MPG backwards, unless you are listening to the Lie-O-Meter. I have 6,942 miles on my 6.7, and I get about 17 highway, and 13-15 city depending on how many times my engine goes into regen, driving habits and all that crap. I have MBRP cool duals, and it did nothing for performance. The only way you will see any improvement with an exhaust mod is to get rid of the Cat and DPF all together, only if you ware willing to sacrifice your warranty and like the color amber on your dash. I still have the stock intake, but I took out that huge plug in the stock pipe. I didnt gain any noticable power of fuel efficiency increase, but I can hear the turbo alot more now. I have a 2 1/2 inch lift and 35 inch tires,from which I gained a MPG from, and 200 RPM less at 65 MPH. I lost a couple MPG in the city though. I have the 6 speed manual. I am about to go in for my first official oil change. Gonna get an oil analysis as well. I will let yall know what I find. Oil change is not due for another 3,000 miles or so, seeing that they changed my oil at 3400 miles due to that oil leak I had.