Originally Posted by Phil57
The regeneration process rarely is needed and has little or no impact on MPG. The 6.7 ISB would not be capable of 360hp and 800 lbs of torque with out the DEF system. The ability to increase HP and improve MPG is because the smog control is done in the exhaust system and takes the load off the engine. There is a huge benefit to DEF and although I hate filling the tank I love that I can get up to 13 mpg with a 40' DP. I also love the fact that the air coming out of the exhaust is as clean or cleaner than what went in. I hate the smell of diesel and with DEF there is no smell.
That's a blanket statement for a wide range of potential situations.
Regen can absolutely affect your MPG, if you are towing heavy loads constantly. The more soot your engine puts out, the more it regens. If you drive unloaded most of the time, then no, you probably won't regen that often.
The 6.7s addition of DEF helped the mileage and reduces the frequency of the regen process, but the regen itself still affects mileage. You are injecting fuel into your exhaust, how can it not?
As with all things, the overall effect just depends on your application.