Hummm. Try this the Ford Build your own truck site.......
http://www50.forddirect.fordvehicles...l=&statetoken=
I see the 4:10 and 3:73 diff being offered. I have the 4:10ls Diff and to tell the truth I would have gone with the 3:73's if I could have found one. We are just short of 10K miles and will do 10/12 mpg towing the 3400 and 15/18 solo both depending on conditions.. The truck full fuel, 2 folks ready to tow weighs in at 9212lbs. The Cummings will get the best MPG..by far..the durimax is about the same as the Ford , so folks who I have met that have the Durimax tell me. I would agree that you will still have plenty of pulling power with the 3:55's .If I was purchasing one now I would probably go that route the price of fuel being what it is. This 6.4L has plenty of pull power.