as far as power goes i have pulled a 36,000 pound load with my truck and i got about 9 mpg doing it the 5.9l is strong dependable and long lasting, but you've heard the saying change is never good at the the start. those 6.7s will come around just give them time. as for now i would advize you to get a 5.9 i mean problems are going to happen but that many at 11,000 miles is pathetic
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2003 crew cab chevy 2500hd 4x4 dmax charcoal grey 85,000 32" procomp xt's and efi live tuned by duramaxin14 13.97 @99mph on a stock trany
yusta have a dodge but we went rollin 
proverbs 13:3 :thumbsup