I have a 96 Ram 3500 cummins 4x4. It came stock with 4:10 gears. I don't need that type of towing capacity any longer and would like the better mileage by dropping to 3:54 gears. My question is, can I just change out the front and rear ring and pinion gears or is there more to it than that?? All comments and suggestions will be appreciated..


Mike
BTW: I recently went to Hankook 305 70 R16 tires and at 70 mph I am tacking 2250 and getting 13 mpg.

 

On my '95 dually that had 4.10s, I had the axles swapped out front and back for another set of factory 3.54 geared ones.
Setting up the gears is best left to guys that do that often was my thought.

 

I "think" it depends on which rear, something is sticking in my head that the 70 and the 80 had different carrier break points, and I thought the 80 was the one that took a different carrier when you went from 4.10 to 3.54.

70's came in Auto trucks and 80's came in duallys and manual trucks.

Yep, just looked it up, 80's break point is 3.73

Differential Carrier Breaks | West Coast Differentials

 

Hmmm..... Seems like excessively low fuel mileage no matter what, and I dont think the cost of swapping for 3.54's would even allow you to regain the loss in money over time, unless you're driving the truck forever. Sounds like there may be other reasons why you're getting such poor fuel mileage for a 12v truck. I'd investigate that one before dumping money into the differentials.

 

You do realize that adding taller tires will throw off the speedo and the odometer as well.
Going from a 30 inch tire to a 33 inch tire with throw things off by 10%.
I am not sure of the height of your new tires but you might getting an extra 1 or 2 MPG "hidden" by the new bigger tires.
You might want to check that as you probably also are speeding around and not know it.

Is the 13 MPG when towing or just running empty?

 

Great point I overlooked. Larger tires will significantly change the mileage traveled on the odometer so even if you're hand calculating (which you should be) your figures will be off. Actually the best way to calculate mileage is with a GPS.

 

I usually try to check if my speedo is off using a GPS or even a street sign that posts your speed over the limit in flashing lights.
Then do the math and add the 10% or 20% it might be off to the ODO reading.
I try to do it with my daughter in our heads as we leave the fuel station so she can use math in a real world setting.