Changing my axle ratio....update

Sky_Blue took the time to type out this awesome reply in the other thread about me changing the axle ratio in my 359. He took the time to help me out so I figure the least I can do is reply.

sky_blue said:

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:
(1)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
(2)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:

(3)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.

(4)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.

(1) Yep that is pretty much where I thought it should run too. Thanks for digging that document up off the web. I was looking for exactly that kind of info but couldn't find it. Basically this truck is geared to run at double nickel with tall rubber. No surprise there since the thing is older than deregulation.

(2) There must be something wrong because when I do a road test in the truck I see a drop of .81 as measured by my tach and my speedo. Speed verified by GPS. I spoke to my trans and diff mechanic today and he said that the OD ratio on a 15 hasn't changed in years. He guaranteed that is was a .79.

(3) 13 OD wit ha final ratio of .73 doesn't get me where I want to be if I am currently at .79 with the 15 OD.

(4) Your hunch is correct. According to the door sticker, it was shipped with tall rubber.

Apparently the trans/diff guy is having a hard time finding a set of 3.42:1 diffs or ring and pinions from Rockwell SQ100. "Antiques" as he put it. He is telling me I may have to swap out the entire rear ends. Not what I want to hear.

 

I asked my trans guy if that was possible.....then a 13 with a .73 gear would get me somewhere. But he said no way all 15 OD's are .79. That's when I took it for a drive and came up with .81 just by going by speed and tach. Margin of error I guess.

 

I found a set of used 3.55 diffs from a different diff shop. He says I can still get new ring and pinions.
There are 24.5 Lo pro's on the truck. As a first step, I may go with a 13 double OD because I don't like the steps in this 15OD. Driving me nuts.

 

I am for sure 1750 (closer to 1800 actually) at 62 mph. Did they make a .87 15 speed? Maybe my diff guy is wrong when he says it has to be .79.