Changing Rears to save on fuel?


Tom or Ray,

I have a Dodge 3500 Dually with a diesel engine, 4x4, six speed manual trans and 4:10 Rears.

The truck gets me around 17 to 21 mpg depending on driver’s mood.

I was told that if I change the rears to a really small number I could get around 27 mpg.

My question is, can this be done? And, would I be crazy if I want to do the work?


Marcos Tesone


You would lose some towing capacity, which I am guessing is why you got this vehicle in the first place. 17-21 MPG in one ton dually is plenty good.


almost forgot. Since it’s 4x4 you will need to change the front gears as well. Figure about $1000-$1300.


You won’t get that kind of fuel economy improvement from the swap.


As others have hinted, you won’t save very much unless you make a big change and then you can’t tow or haul very much with it.

As far as doing it yourself, I have never done it, but have been told that it is not a job for the neophyte.


The guys are all correct.

Just let me add: the weight of the dually will prevent higher mileage.

Like was mentioned both the front and rear diff have to match: it is a job for a very experienced drivetrain specialist as if the diff is not installed and ‘fine tuned’ to specs you will wind up with howling differentials and finally: You’re looking at about a grand PER diff.

Short answer? No.


milage and dually=oxymoron.


Sort of a dumb question but / Can you put on tires with a larger diameter ?
If you could go up an inch what would that do ?


ask the math guys on this site.

I can figure it out ,but to be honest I do not care enough to do the math for you.if I were building a real car then I would.

you are not even the OP(or are you?)


You won’t be seeing 27 or anywhere even close. I have a half ton chevy 4WD and a 4.8 gas engine and my mileage is 17 on the highway. Maybe you can be happy with the diesel except for the price of fuel. You could, by some accident, get worse mileage too. The other option would be for the price of fuel to go down. We all know that it could happen.


I don’t know what size tires are on the truck. Assuming that they are 20 inch for ease of calculation, increasing them by 1 inch would be a 5% increase. I hope that you are not thinking that a 5% increase in diameter will yield a 5% increase in fuel mileage, because it won’t. You would be lucky to get half that and, again, your towing capacity will be lower.


If you put a 2.58:1 rear in it, you could bet close to 27 MPG (mathematically). But you would not have a good low gear for your 4x4 moments or any towing you amy do. If it’s a work truck and you load the bed up, don’t do it. You won’t get around the work the site very well.


The highest gear Dodge put in those was a 3.55. I wish they still had it. The highest they have now is a 3.73.

It can be done, but you need someone who knows what they are doing and has the tools to line it up right. You’ll have to change both the front and rear differential and it’ll be expensive, about $4000 for both. The cheap way would be to find a wreck with 3.55 gears and get the axles out of it and swap them whole.

I really would like to convert my 2500 to 3.55 gears and I don’t believe it would hurt the power a bit although I’m not towing over 10,000 lbs, if you are, it might be a different story.



What is the engine RPM at 60mph?