Well, where should I start. I bought an ISX15 in a 2005 KW in June 2015. Used to get 6.5 mpg if I drove it like I stole it and 7.5 if driving like grandma. Sept 10, 2015, I broke a crankshaft. Replaced motor with a brand new motor in Oct 2015. Mar 2016, replaced EGR, Apr 2016, replaced EGR Actuator, May 2016, I replaced the head because it cracked across all 6 cylinders. June, rear main seal and I still can not get over 5.5-6 mpg. Now it's at Pittsburgh Power and they are trying to get it back to where it should be. I believe that my next truck will have either a Cat or a Detroit in it. My Cat (2WS) used to get 6 to 6.5 all the time and it was in a freightliner Classic. Detroit 12.7 was in an International 9400 was getting 7.5 to 8 all the time.