I had the Hypertech on my Duramax (that I sold for an 09 6.7). I had guages before and after adding it. The power was significantly up but so was the EGTs. Without the pyro I think I would have smoked the motor. Mileage didn't change. I am not adding anything to my new 09 as it runs fine pulling my 18k RV trl and I don't want to mess with the warranty.