on a new truck, 2006. with the auto you have either 3.73's or 4.10's. the 4.10's would be used more for pulling larger loads. the Manual only offers 3.73's since the final drive ratio of the G56 manual tranny is so high. in 04 4.10's were offered for the manuals since the final drive was low enough to keep the cummins from turning too high of an RPM and the fuel mileage being in the toilet.

if you plan on towing below 10-12K the 3.73's will do just fine on the auto, but anything over 14K the 4.10's on the auto would be good to have. however, there are plenty of people that pull over 14K with 3.73's without a problem.

 

I got the 3.73 gear and at 70 im turning about 1900 and 75 about 2200.