looking to change my axle ratio

that is not objective data analysis that I was inquiring for. That is anecdotal stuff that doesn't state much of anything. Heck, i walk on up the hills all the time past typical company spec'd trucks irregardless of trans and rear ratios. To make a objective analysis, one has to have the identical spec's except for the diff ratios. i.e. you would need all things equal, including the trans, and compare power to the ground using the different ratios. It always seems to be interesting, that serious dyno testing of engines, in chassis situations, is done using the direct drive on the transmission and not the overdrives, to enable the dyno to give a more accurate reading on the power and torque.

Now, like I stated, I would like to see some real solid data, and not simply "I can blow them off on hills" stuff to prove that running in overdrive using higher number rear ratio will put more actual power to the ground compared to running direct. If this were the case, that data would be readily available.

But in the final analysis, those that are not open to other ideas will not be swayed in the least. I am beginning to think this whole thread is just a way to play head games and not view things from a true testing measure which is the only way to realistically approach the issue.

And for all your "real world experience" there are some of us that have as much or more real world experience with the setups we use. Still, no one has proven a thing to me that they can out pull me on a hill when we have the same gross weights, the same trucks, the same engines, the same tires, the same trans, and the only thing different is the rear ratio with them running OD and me in direct. If I was losing so much on hills, I would sure be the first to head into Des Moines and get different ratios put in the diffs. Have not had a need to do that. Now if my operation changes, that may be something I will do. For now, the 18 and 2.64's do a pretty good job, so there is no need to spend anything to change. No one is calling my on my CB and asking why I am falling back on a pull.

 

I didn't read the 25 pages before this so sorry if this has been brought up before. That 15 is probably a single overdrive .86 or .87 final ratio. Swapping to a 13 speed double overdrive would get you roughly 10-12 more mph at the same RPM. You could also go with an 18 speed if you're in a hilly area or drive over 80k. I'm a fan of high 3, low 4 ratios myself coupled with a good OD tranny, and an older cat like yours doesn't like to be lugged down to much less than 1400 RPM so this might be a worthwhile alternative.

If you insist on changing the ratio, I'd go with a 3.55 or 3.58. As a second choice no smaller than a 3.42.

 

Still waiting.

 

really that was for alaln5oh. didn't mean to offend you. Never said anyone didn't do real trucking, I said you can take all that crap they tell you will work on paper and apply it in real world trucking and it don't always work. I have always recommended anyone that ask to read your post on fuel mileage. But if your going to tell me your 2.64s pull as good as 3.55s up a hill all other things being equal im not sure now.

 

I can assure you this:

If you drove a truck for 10 years and it had 3.55 gears with a 10 speed B ratio overdrive, and I swapped them out to 2.64 gears and a direct drive 10 speed without telling you. I promise you wouldn't notice a lick of difference. The ratios in the transmission make up for the lack of ratio out back. I don't know how many times we have to repeat this.

If you cloned a driver and had two of those trucks side by side, one with OD and one direct drive, the trucks would behave exactly the same. Take off from a stop, shift points (engine rpm + MPH), pulling up hills, etc..

 

I did a little bit more thinking and decided to do a bit more research. I decided to look up the recommended cruise RPM by cat. I couldn't find one for a 3406B but I did find one for a 3406C. I will base my post over the specs for the 3406C because that is where I have the most complete information.
3406C specs
C15 specs (for reference)
C16 specs (for reference)

From the 3406C spec sheet:
For the best balance of performance and fuel
economy on the 3406C, spec axle ratios and tire
sizes to obtain:
1475 rpm @ 60 mph

Subject to the following:
Maximum recommended engine speed at
cruise — 1550 rpm
Minimum recommended engine speed at
55 mph cruise speed — 1350 rpm

From the Eaton Road Speed Calculator:

Engine RPM 60/(Axle Ratio x Tire Revolutions per Mile x Transmission Ratio)= Top Road Speed

Also from the road speed calculator, 24.5 lopro tires are 498 revs per minute.

Now that we've got that out of the way, lets do a little math to figure out what the overdrive is on his current tranny.

1750*60/(3.90*498*X)=62
105,000/1942.2X=62
105,000=62(1924.2X)
105,000=120,416.4X
105,000/120,416.4=X
X=.8719
So barring any mistakes in my math, he currently has a .87 single overdrive transmission at the moment.

This truck is definitely not at Caterpillars recommended RPM.

I set up a a small program in excel where I'd plug in the ratio and it'd spit out an RPM. I also set the speed at 62.14 as that is what the OP said was his cruise speed currently and caterpillar recommends to gear the RPM to your cruising speed.

A 3.73 ratio change and no transmission change would put him at 1675 RPM at that same 62 MPH
A 3.70 ratio change and no transmission change would put him at 1660 RPM at that same 62 MPH
A 3.58 ratio change and no transmission change would put him at 1607 RPM at that same 62 MPH
A 3.55 ratio change and no transmission change would put him at 1594 RPM at that same 62 MPH
A 3.42 ratio change and no transmission change would put him at 1535 RPM at that same 62 MPH
A 3.36 ratio change and no transmission change would put him at 1508 RPM at that same 62 MPH
A 3.25 ratio change and no transmission change would put him at 1459 RPM at that same 62 MPH
A 3.08 ratio change and no transmission change would put him at 1383 RPM at that same 62 MPH

With this information, and given no option to change the transmission, I would recommend a 3.08, 3.25, 3.36, 3.42, and a 3.55.

Now lets look at things if we swapped to a .73 double overdrive 13/18 speed:

The stock 3.90 ratio and a change to a .73 final drive would put him at 1469 RPM at that same 62 MPH. I don't know about you guys, but that seems like a perfect solution.
A 4.10 ratio change, and a change to a .73 final drive would put him at 1544 RPM at that same 62 MPH.
A 3.73 ratio change, and a change to a .73 final drive would put him at 1405 RPM at that same 62 MPH.

Let's see what happens if we switch to a direct 1.0 final drive transmission and change out the ratios as well:

The stock 3.90 ratio, and a change to a 1.00 final drive would put him at 2012 RPM at that same 62 MPH.
A 3.55 ratio change, and a change to a 1.00 final drive would put him at 1831 RPM at that same 62 MPH.
A 3.25 ratio change, and a change to a 1.00 final drive would put him at 1677 RPM at that same 62 MPH.
A 3.08 ratio change, and a change to a 1.00 final drive would put him at 1589 RPM at that same 62 MPH.
A 2.85 ratio change, and a change to a 1.00 final drive would put him at 1470 RPM at that same 62 MPH.
A 2.79 ratio change, and a change to a 1.00 final drive would put him at 1439 RPM at that same 62 MPH.
A 2.64 ratio change, and a change to a 1.00 final drive would put him at 1362 RPM at that same 62 MPH.

If switching to a direct drive transmission (or driving in direct gear on that 15), I'd recommend a 2.64, 2.79, 2.85, or a 3.08 ratio.

As a recap, you could switch to a 3.08, 3.25, 3.36, 3.42, or a 3.55 and keep your current transmission, swap to a 13/18 speed, or switch to a 1.0 final drive transmission (or drive in direct) and also switch to a 2.64, 2.79, 2.85, or a 3.08.

Myself? I'd swap to an 18, keep that super strong rear end, gain a lot of flexibility in how you can drive the truck, and very few if any drawbacks. A slightly less ideal but still a relatively good option is to swap to a 3.55 rear end and drop 156 RPM in every gear, or switch to a 3.42 and drop 215 RPM on every gear.

For a truck like this, I don't recommend the swap to a direct drive and low gear rear end. The truck is designed to produce it's peak RPM and Torque much higher than say a 12.7L detroit or a new DD15. It's just not good for the engine. Were this truck 15 years newer with a detroit I might recommend doing such a swap, but you have to do what this old kitty wants if you want her to behave.

The choice is ultimately up to you however, so take this information however you like. All of us on here have experience in slightly different ways and slightly different areas, so what worked for one might not work for the other.

I also have a hunch that this truck originally came with 24.5 regular sized tires, as with that tire size it'd run at 1673 RPM @ 62.14 MPH everything else the same, and 1616 RPM @ 60 MPH. Still on the high side, but more acceptable.

 

Everything from the output of tranny to the axles is slower? If the trucks are running the same speed with the same tire the axles are turning the same revs per mile. Wheels bolt to hubs, hubs to axles. Axles spline to the carrier thru the axle gears. Pinion is still rotating slower true, the carrier however is not turning slower.

 

"TO" the axles, not including the axles. You are correct about the ring.