I have a AAM 11.5 rear end with 3.73 gears and I want to change to 3.42 gears. I would like to do the work myself. (while I'm at it I also want to put in a Eaton Truetrac Limited slip). Does anyone know where I can find a video that shows how to do this? I have looked on youtube but can't find exactly what I need.

 

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I assume you believe this will reduce fuel consumption. It won't. Your truck is about as aerodynamic as an outhouse. Speeds above 60 mph really effect fuel consumption. Decreasing engine speed will not decrease the drive requirement. You will simply use heavier throttle settings at the lower engine speed. Adding that camper will only make your drag worse. In point of fact, going to 4.11 gears might actually save more fuel than going the other way. This has been my personal experience, not here say from others. I have run gear ratios of 3.55, 3.63, 3.73 and 4.11. I can tell you the factory got it right with 3.73 for general use. For higher loads like campers and towing, 4.11 is the best. Going to taller gears would only work if you could change the aerodynamics.

 

Tru-trac is sweet. You'll love it.

 

Oh so true. These motors have to stay in the sweet RPM range to maintain fuel mileage and drivability especially towing. A few hundred rpm can really effect the performance. I have attached a tire size - gear ratio - speed chart that may help you with your gearing choice. You can change the gear ratio at the top of any of the charts to help you see the rpm difference vs speed. I have found that 1800 - 2000 rpm towing gives me the best fuel mileage, keeps the turbo spooled & EGT down. Hope this helps.

 

It looks like you are getting good replies here... I think they are correct in telling you, you're going the wrong way. You should tell us what RPMs are you at when running down the highway. What are you trying to achieve?
Want to increase millage look into the hypermilers. (sp)
Also remember, cold air (intake) gives power, warm air gives mpg.
Guys do it with adding another front airdam and stuff. Most of the issues are the air going under the truck. There is also issues with air hitting the nose, the width, the height, windshield... see a theme here? It's a shed with wheels.

 

The Eaton Tru-Trak is a modified Gleason worm gear. A limited slip differential without clutch packs. The stock LSD in these trucks is the same design. If your truck already has LSD, then swapping to the Eaton won't change anything. Except make your wallet a lot lighter.

I had a Ford 8.8 differential rebuilt and a new Eaton put in a couple of years ago. It was about $850, and the builder told me that the new Eatons were not nearly as well made as they used to be. Fortunately for me, he had an older one sitting on the shelf.