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I would much prefer the 3.73 rear and 6 speed to the 4.10. My father has the 6.0 with 4.10's in his 2500HD and it is a fuel HOG!! With the 4.10's 4 speed tranny with OD locked out, you will be over 3,000rpms at 60mph.
I've got a 2003 2500 Suburban with the 4.10 rear, 6.0 engine, & quadrasteer. Boat and trailer combined weigh around 7,500 - 8,000lbs. Tows like its not there, mileage towing is about 8.3mpg at 60 to 65mph. Mileage unloaded averages 11-12mpg .
I hope this helps, I have an 08 Denali w/ 6.2 liter and 6speed auto. I tow an Onlsow bay 23T including trailer is 8500#. I get 8-10 towing and 17mpg empty on road trips. Avg. around town is 14-15. The 6.2 is very strong at 403hp and makes the truck very sporty. The Deanli is somewhere between the 1/2 and 3/4 ton vehicle from what the dealer says,,, it's tow rating is 10k but it still has 6 lug wheels and he stated its on the 3/4 ton chassis (???) I do know that the airbag system and the brakes are different than both of the Burbs. Overall I love the Denali, I just wish I could get a Deisel in one of the GM suv's. FWIW, I have a 08 2500 crew cab with the Dmax and I get 12 towing and 19 highway. I think we are in the same boat....Wifey will not go with me and our 3 boys on long trips...The fighting starts before I get a chance to check the trailer lights...Got to seperate them with a 3rd row!!! Good luck
Low Country Native
23T Onslow Bay
I have a '07 2500HD Silverado with a 6.0. 6 speed, with 3.73s. Towing 12,000lbs @70mph is ~ 8mpg. Unloaded it gets ~ 15mpg. I love the 6 speed, its a great transmission. No more 5000rpm downshifts going up a hill.