It is so hard to compare engines because there are so many ways to set up a truck and so many applications a truck can be used in. Also, studies have been shown that there is at least a 20% difference in fuel economy between the best and worse drivers under identical situations. Cummins covers the topic of fuel economy quite well in the PDF document at the following address:
http://www.everytime.cummins.com/ass...Whitepaper.pdf
While I’ve never owned one, Detroit has always had a good reputation when it comes to fuel economy (one of the primary reasons a lot of big companies run them) and they are easy to find service for out on the road. My 2003 CAT C-15 had a lifetime average (in the 645k miles that I put on it) of 6.2 mpg. My 2006 ISX was averaging 6.7 mpg before I unplugged my EGR, and since has gone up to 7.2 mpg in mostly winter driving, which always hurts economy.
Finally, keep in mind when talking to people about their own fuel economy that there are several ways to derive their economy numbers. Some will simply look at their ECM report or go by what their computerized in-dash display tells them. Both of these methods will yield inaccurate data. The only way to accurately figure and track fuel economy is by the “at the pump” numbers. Take the miles you have driven since your last fill-up and divide by the gallons required to fill your tank(s) back up. I do this with every fuel purchase from the first day I buy a truck and keep the data forever in an Excel spreadsheet so I can analyze trends that might take many tens of thousands of miles to develop.
