Ram 2500 Cummins the most fuel economical

I don't believe you will find one brand diesel having a significant advantage over another in mpg, perhaps in different model years maybe, but new, not so much. As was already stated, driving habits, loading, wind, etc all have an affect. If you're trying to make a new truck purchase decision on this factor alone, you may be focusing on an unimportant issue. I believe all the big three diesels are very capable haulers and darn nice trucks. I've always been a Cummins guy having just purchased my 4th one. Would still have #3 if I didn't need to upgrade to a dual wheel, it only had 103K miles on it when I traded it. Just saying...