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I have a 2012 F350 SRW 4X4 with a 3.31 rear axle. I have no problem whatsoever towing my 14,500 lb toy hauler on the highway or through mountain passes. I'd go for the 3.31.
It's not as easy as you'd like to answer your question.
Dually, 3.73
SRW, 3.31 with 18" wheels
SRW, 3.55 with 20" wheels
If you're getting a diesel, just get the tallest gear you can which is what everyone is saying. It might be different now but I think the best you can do in a dually is 3.73 and the SRW trucks are different with the wheels they put on at the factory. I'm pretty sure to get 3.31 you have to have 18" wheels and some guys go and swap 20" on later.
With the 6-speed tranny, gears are not that big a deal on SRW trucks. I do like the 3.55 and I do run highway speeds a lot. Any legal highway speed you are still just cruising with 3.55s. I had 18s with 3.55s and 20" with 3.55 and one truck had 20s with 3.31s...
Where you *might* start to have issues with the 3.31's is if you start going to a lot larger than stock tires.
Well got it, engine seems nice. Powerful and smooth.
Not as many option changes as I though from 08.
In fact my 08 was nicer in some ways , but I'm liking this engine and all the tow goodies.