Hi all
I am a new owner of a 2011 SD with the 6.7l and was wondering how to find out weather my truck came with the 3.55 or some other gear ?

 

does it 20" wheels or 18"? if it has 20'2 more like has 3:55
also try putting you vin # at the end of this webpage

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If you look at the build sticker at the base of the drivers rear door pillar you'll find an axle code. I can't tell you what they all mean but mine has a 3.55 w/ELD and the code is 3J. -- EDIT: Just looked it up. Assuming you have a non-dually the codes are:
31 = 3.31 non limited slip
3H = 3.31 ELD
35 = 3.55 non limited slip
3J = 3.55 ELD

 

3:55 or the ratio is the number of times the drive shaft will turn to one revolution of the tire.

So you are not sure jack up the truck turn the tire once. Count the how many times the drive shaft turns. On stock tires

The higher the number the worst the fuel mileage, the lower the better fuel mileage.

Also the higher number the better for towing, lower not as good for towing

 

3.73 gears are 3L for the axle code.