Need advice for gear ratio

3:36 or 3:32 I wouldn’t want drop much more than that because your gona get out of your comfort range on the low side of RPMs
I also have my doubts your going to gain enough in fuel mileage to warrant the cost of the change JMO

 

Without getting to far into the weeds, everything needs to be in balance. Yes you can change ratios, but that is not free. You may gain mpg, but depending on the engine, constant lugging ‘may’ shorten the life of that motor. Wrong ratio, and you may find yourself down shifting all the time at the slightest hill. You may need to change parameters on the fuel pump/ecm/computer, etc. You state you have a Cummins engine, a Cummins what? There’s all kinds that went into KW’s back then.
The best thing to improve mpg, is pretend there is an egg between your foot and the accelerator.

 

Why not just run 65 and save the money you'd spend on a gear swap? As Last Call said, the cost of a gear swap (unless you're doing it yourself) will probably not outweigh any savings in fuel economy.

And it depends on where you're running as well. I wouldn't want 3.36 rears doing what I do, which is local tanker work in central PA. I actually ran that setup in a previous truck. 2014 Western Star glider. 3406 Cat 550 hp 13 speed with 3.36 ratio. I was getting just over 5mpg in summer. And as low as 4.2 in winter.

Next truck was another Western Star glider. 6nz Cat, 550hp, 10 speed, 3.73 ratio. That was a 6+ mpg truck in summer.

Current ride is a 6nz Cat 550 hp, 18 speed with 3.55 rears. Again, I'm in the low 5's in summer and mid 4's in winter.
So it really depends on where you run, what you're hauling and the speed you wanna run.

 

This sums it up. Current rig is a 455 paccar that started life with a 1450 13 speed (dont ask, last owner did it and im not sure either) and now has a 2050 18 speed, with 3.28 gears. I get around 6.5 average pretty much all the time and my truck is a blasted slug. You do NOT want a low gear ratio if your going over anything bigger then a mole hill.

a buddy running a slightly older 579 with a C15, 2050 18 and 3.55s is getting 7 doing simular loads in simular areas.

Gear ratio swaps arent really going to do much to improve MPG. Adding stuff like skirts on your trailer, diffrent tire size or wings to the back and such are going to give you better gains for far less money if its that important. Or better yet just back her down yourself and keep the power for when its needed. Also where you run is going to have a HUGE effect. If your climbing mountains all the time your going to average lower MPG, if your running kansas flats all the time you will see higher MPG. Moreso if you underpower or overpower your rig for the application.

Thats really all there is to it.

 

Doesn't work that way. The only thing you'll gain with a lower number( higher ratio) is less rpms, or "motor miles", but trucks, as mentioned, need to be in harmony. Diesels have an "operating range", meaning it's most efficient at a certain rpm. While it seems raising the ratio, lowering the rpms would be better, but actually, you may get worse mileage. It's been what, 40 years, and trucks even with the fancy fairing crap, have never got any better mileage now than they did 40 years ago. I say a huge conspiracy by the oil companies,,

 

There has been a lot of good answers given to your question there’s nothing I can add that they already haven’t told you .
I’am not going to tell you I’am a expert on what is perfect but I do know my way around the basics . I’am going to tell you this that I think your chasing some what of a myth about ( just change the gear ratio) to gain fuel mileage. Since you asked me about why not drop to 3:08 here’s what I’am fixing to tell you on the limited info you have posted
1 … what is your exact engine horse power .
2….what size tires are you running
3… what are you pulling
4….where do you run . are you running in the Midwest or east or west or in the mountains or plains or both ?
If you drop to a 3:08 it’s gona be harder to get rolling from a stop which is harder on your driveline and motor, also your going to be shifting more often in hills and to be honest it’ very well could throw you outa of a nice cruising range in other words at 65 mph you might be lugging your motor alittle to much in 18 th and you might find that your still at 1750 or 1800 RPMs in 17th now that’s just a guess with out knowing all the details that might be the issue at 70 or 78 mph Again I honestly don’t know with out knowing all your details I just giving you a general example I have seen alot of guys very disappointed by making ratio changes going off the mind set lower is better. Again thinking you going to gain huge fuel mileage I think your overly optimistic.
Just a ball park figure you going to have about 5.000 in making this change another factor I don’t know is the total mileage on your truck which could play into the cost also the make of your rear axles will determine it also
I could waste both our times telling you the story about why I went from 3:70 gears to 3:36 gears and why I was so adamant about getting that ratio and exactly how costly it was . But my reasons were completely different that yours or most other people’s and I wasn’t looking to gain fuel mileage .I also have a completely different goal and I would be comparing apples to oranges.
All this said I hope you take serious thought in to everyone else’s advice because it has all been good advice and weigh you final decision heavy because what ever you decide it could be very costly in the end Best of luck to you

 

Look at it like this. You have a boost gauge in your truck. With your 3.55 on a slight incline, running 70 @ 1650, lets say your pulling 23lbs of boost.

Now you swap in 3.08, running 70 @1450, same incline. Will you be pulling more boost, or less? And then why?

Learn that and you can understand why it's not so black and white. There's extremes on both ends, and a happy medium.

 

I'm almost convinced the only way a gear swap for fuel economy makes sense is if you need to replace a rear anyway due to failure. Or else you have something like a 4.56 ratio