I prefer cummins. Detroit is taking a nose dive as Frieghtliner/Benz phases it out to be replaced with the Benz motor. The Cat I apsolutely destest. Its a great motor if you are pulling oversized loads/overweight loads. But for regular straight truck, 18 wheel truck, the fuel mileage doesnt warrant it. I have driven all 3. And no, the rumor about the detroit having to be rebuilt at 300k is untrue. All have the issues when new, and are generally worked out at 100k.
My current Cummins (2003 Volvo 770 Semi with 358,000 miles, ISX 600 turned back to 450, Autoshift 10 speed) gets anywhere from 8.3-11.7, with an average of 9.2 depending on how hard I am on the pedal. I can say it pulls hills better than Any truck I have driven.
My last truck (2001 Volvo 770 Semi with 476,000 miles, Detroit Series 60 430/470, Autoshift 10 speed) got 6.6-7.2.
The truck befor that (2000 Volvo 770 Semi with 424,000 miles, Cummins N-14 Celect-Plus 430/470, Autoshift 10 speed) got 7.2-7.7.
The truck befor that (98 Frieghtliner Century Class Semi Cat, Dont remember what kind long time ago for me, 370,000 miles, autoshift 10 speed) got from 3.2-5.1. Just apsolutely Horrid on Fuel Milage. Which is the prime reason I got rid of that truck.
If you are in it to make money, go with a Cummins or a detroit. If you want to climb hills and watch your fuel guage drop at the same time, get a Cat. My money is with my cummins, and probly will be for a long time.... Well until I get into the cargo Van thing with my wife...
And yes in case you didnt notice, I love Autoshift trucks, and Volvo, wont drive anything else in a big truck. I have driven all of em in my career. None ride as nice, have ease of use, have as good as visability, and give you as much storage room off the show room floor. All my 770 have taken a 21 inch TV in their TV hole without any modification. None can hide my Fridge, Microwave, and still give me as much storage space.
Thats my story, and I am stickin to it.
Brian