Rear dif gear recommendation?

[OhioSkoolie]

Hello All, I have a 2006 International Ce300 25' 6 window dog nose school bus that has a 7.6 L dt466e

The bus weighs 16,000 lbs and had a GVWR of 27500.

I currently have a rear differential that has a 5.57 ratio.

I would like to change the carrier assembly to one that is better suited for freeway driving as it's a road trip vehicle with friends. Road trip averaged around 7-8 mpg and I would like to get it slightly better at highway speeds.

What ratio would be best without affecting performance too much. I've seen both 4.44 and 4.7 thrown around

Thanks in advance

 

[PorchDog]

What transmission do you have, and what is the gear ratio in the highest gear? I would suspect you have an Allison 2500 and it should have an overdrive, or maybe two? That and your tire size would strongly influence your gear choice. I have a DT 466 in a bus similar to yours, and I have one overdrive in 5th gear of 0.75 (or 0.74) and I re-geared from 6.14 to 5.38 to get the RPM's down at cruising speed...I currently have 10R22.5 tires, and when I replace them it will have 11R22.5s-- because the 11R's are much more available if you need one... that will lower the ratio some too...

If you google Spicer Gear Ratio Calculator-- this is a very nice tool to do some calculations.

10R22.5 tires are about 40.3 inches in diameter, and 11R22.5's are about 41.8 inches in diameter.

 

[OhioSkoolie]

What transmission do you have, and what is the gear ratio in the highest gear? I would suspect you have an Allison 2500 and it should have an overdrive, or maybe two? That and your tire size would strongly influence your gear choice. I have a DT 466 in a bus similar to yours, and I have one overdrive in 5th gear of 0.75 (or 0.74) and I re-geared from 6.14 to 5.38 to get the RPM's down at cruising speed...I currently have 10R22.5 tires, and when I replace them it will have 11R22.5s-- because the 11R's are much more available if you need one... that will lower the ratio some too...

If you google Spicer Gear Ratio Calculator-- this is a very nice tool to do some calculations.

10R22.5 tires are about 40.3 inches in diameter, and 11R22.5's are about 41.8 inches in diameter.

You are correct sir, it's a 5 speed Allison 2500, currently has newer 11r22.5 tires on it.

 

[PorchDog]

Maybe someone who has actually done this with an Allison 2500 will chime in. What you don't want to do is go too low such that it cannot stay in the highest gear-- and is constantly hunting back and forth between gears...

 

[Dbacks2k4]

Depending on the build date, that Allison 2500 might be able to upgrade to 6th gear without requiring any hardware changes, just a TPM re-flash. There was a cutoff date mid-year 2006 I believe. There's a thread post about it somewhere.
I did the re-flash on my Allison 3000 and saw my RPMs drop by about 300 at different highway and freeway speeds.
Pretty sure I've got a 5:29 rear diff, but I have a full 40' bus and the bigger trans.

 

[OhioSkoolie]

Depending on the build date, that Allison 2500 might be able to upgrade to 6th gear without requiring any hardware changes, just a TPM re-flash. There was a cutoff date mid-year 2006 I believe. There's a thread post about it somewhere.
I did the re-flash on my Allison 3000 and saw my RPMs drop by about 300 at different highway and freeway speeds.
Pretty sure I've got a 5:29 rear diff, but I have a full 40' bus and the bigger trans.

Since mine doesn't have the digital shifter I do believe it to be built before that was possible. I've attached a picture if that helps of the plate on the side of it.

 

[nikitis]

Maybe someone who has actually done this with an Allison 2500 will chime in. What you don't want to do is go too low such that it cannot stay in the highest gear-- and is constantly hunting back and forth between gears...

This is true. I had a conversation with @cadillackid once about this, and he stated it should be safe in the 4.4x+ range, below that and you are risking things with it lugging.

Most people historically chose 4.78 as the sweet spot, but lately some have been going in the 5.xx's as not to chance it. I'm on 4.78 myself but I have the crappy 545 trans.

 

[Timeline]

My 38' bus has the same engine and tire size with a 5 speed 2500 with 5.29 rear end. It gets up to highway speed pretty quickly. I like to run at 1750 rpm = about 63 mph.

 

[TaliaDPerkins]

1550​	Engine RPM​	​
0.65​	6th / final gear​	​
3.91​	differential ratio​	​
40.1​	10R22.5 tire diameter​	​
3.1415926536​	Pi​	​
125.977865409​	inches per revolution​	= Diameter * Pi​
2384.6153846154​	Xmsn output rpm​	=Eng RPM / Final Gear​
609.8760574464​	axle rpm​	= Xmsn output / Differntial Ratio​
76830.8838811228​	inches per minute​	= Tire Diameter in inches * Axle RPM​
6402.5736567602​	feet per minute​	= Inches per min / 12​
1.2126086471​	miles per minute​	= Feet per min / 5380​
72.7565188268​	mph​	= Miles per min* 60​

If it helps ( I hope ) there is an example of speed calculation by transmission final gear ratio, differential gearing, and RPM and tire size (from manufacturer).

These websites were useful.

https://www.tritrack.net/horsePower.html

https://www.blocklayer.com/rpm-gear

I calculated the HP needed from my probable (wild ass guess) drag coefficient and weight for the speed I desired.

I compared that HP to the chart of HP and torque vs RPM from the engine manufacturer.

My goal was for the lowest RPM still generating about 120%+ of the HP I needed, to have an RPM and gearing still making the highway mph I desired. I assumed about 95~100 sq ft for my roof raised skoolie.

Hope is that on the level and towing, running at ~83%-90% throttle at minimum RPM the exhaust gas temp isn't creeping up past 1200 or so. If it does I gear down

I'm running a 330HP Cat 3126B with an MD3060 and a 3.91 differential.

​

 

[Dbacks2k4]

Since mine doesn't have the digital shifter I do believe it to be built before that was possible. I've attached a picture if that helps of the plate on the side of it.

View attachment 1953754

The digital shifter was on the 3000 series. The 2000/2500 series were all T-handle. Youd have to decode the date stamp on the trans plate to get the date and then lookup the month in 2006 it changed, I wanna say it was June or so but don't quote me on that. Your Trans looks like maybe it was rebuilt at some point which will make it harder to tell if it has the correct valve body assembly or not.

For further reading...

Allison 2500PTS 6th Gear Unlock

I got to talking to an Allison dealer about unlocking 6th gear on a 2500PTS. While it can be done, the process is bureaucratic and not guaranteed. There are two paths you can take: Allison and OEM of bus authorization PCM Performance (pcmperformance.com) Before you start, any Allison...

www.skoolie.net

 

[OhioSkoolie]

The digital shifter was on the 3000 series. The 2000/2500 series were all T-handle. Youd have to decode the date stamp on the trans plate to get the date and then lookup the month in 2006 it changed, I wanna say it was June or so but don't quote me on that. Your Trans looks like maybe it was rebuilt at some point which will make it harder to tell if it has the correct valve body assembly or not.

For further reading...

Allison 2500PTS 6th Gear Unlock

I got to talking to an Allison dealer about unlocking 6th gear on a 2500PTS. While it can be done, the process is bureaucratic and not guaranteed. There are two paths you can take: Allison and OEM of bus authorization PCM Performance (pcmperformance.com) Before you start, any Allison...

www.skoolie.net

Hmm I will have to crawl under it tomorrow and see what type of plugs the TCM are utilizing the one vs two.

I do have the NEXIQ3 usb adapter, Servicemaxx, Wabco Toolbox and Allison DOC software so is there any information on performing the unlock yourself vs sending if off for $400? I was able to remove the governor and enable functional cruise control myself which was a nice feeling.

Would we happen to have a part number for the rear diff? On demand online is not the most detailed and Im still looking to get OnDemand / ISIS software that I hope will be a bit more in-depth in looking up the part number for a possible diff replacement. ETN0510914 is what Ive been able to find so far for 4.88; ETN0510916 I believe being what I have 5.57.

 

[Dbacks2k4]

Hmm I will have to crawl under it tomorrow and see what type of plugs the TCM are utilizing the one vs two.

I do have the NEXIQ3 usb adapter, Servicemaxx, Wabco Toolbox and Allison DOC software so is there any information on performing the unlock yourself vs sending if off for $400? I was able to remove the governor and enable functional cruise control myself which was a nice feeling.

Would we happen to have a part number for the rear diff? On demand online is not the most detailed and Im still looking to get OnDemand / ISIS software that I hope will be a bit more in-depth in looking up the part number for a possible diff replacement. ETN0510914 is what Ive been able to find so far for 4.88; ETN0510916 I believe being what I have 5.57.

No, even with Allison doc you can't reflash the tcm, you need a configuration file that matches all the parameters. Lookup Charlie Ball, he's who did mine - extremely reasonably priced and excellent customer service. Allison dealers won't do it without a sign off from international who rumor has it tends to ghost anyone who asks, probably because of liability combined with there being no money in it for them.

 

[desrtdog]

Allison dealers won't do it without a sign off from international who rumor has it tends to ghost anyone who asks, probably because of liability combined with there being no money in it for them.

Same with Blue Bird x it is a non starter trying to get approval to open 6th on the TCM. I used Vektor Tuning Systems (NY) …Alex is the guy’s name… recently sent him a 2nd (spare) TCM to program for 6 speeds

 

[PorchDog]

Sometimes the gear ratio is stamped into the end of the pinion gear where the big nut screws on.

 

[nikitis]

If it's international there's a white sticker plate with the gear ratio on the front of the axle passenger side. It's often completely covered in grease and you won't even know it's there. If you wipe the area down it may surface and list your rear axle ratio.

Also about the digital shifter. It's not required on a MD3060. Cadillackid and I were working on transmission upgrades for the 3box's, and confirmed that you can use even an older 3 gear drive handle shifter on them. The TCM understands it's a manual handle shifter and not a digital one. You do miss out on 4th manual shift, but can still do 1,2,3,D,R. D will take it up to 4th, 5th and 6th gear still assuming it 6th was unlocked on the TCM.

 

[Mr P]

To the OP: What is your current "top speed"? What is the engine RPM at that speed?

 

[OhioSkoolie]

To the OP: What is your current "top speed"? What is the engine RPM at that speed?

Since I removed the software gov, its been up to 80, cruises around 70 at 2400ish rpm which matches the calculator for fifth speed transmission, 11r22.5 tires and 5.77 gear ratio.

Its starting to look like 4.88 with the sixth gear unlock *if even possible on my unit haven't been over there yet today* would bring me down to 1755rpm for the same speed with a 4.88 and sixth gear. Which is quite a drastic improvement in fuel economy and noise.

With only the 4.88 rear diff change and no sixth gear that would still have me up around 2030rpm at 70mph so a bit less helpful without the sixth gear.

 

[OhioSkoolie]

Well unfortunately it seems the 6th gear on the transmission isn't going to be as simple as a reprogram. I do believe with the TCM having two separated plugs going into it I have the gen 3, which would make sense with the build date of my vehicle being the first half of 05. Added photos of TCM and connectors.

With that being said, I now know from @nikitis I shouldn't really go below 4.4* but I'm not quite sure what the options would be, looking for something lower than 4.88 but higher than 4.4. I have a local Vander haags, but just not sure what list of options would be available for my rear air ride axle.


4.44 Gear ratio would be me around 1840 rpm cruising at 70 with the 5 speed.

 

[Mr P]

Search for other recommendations and see if they agree with Nikitis. I would rather trust someone who has done it before...


Oh, what rear axle do you have?

 

[Mr P]

1550​	Engine RPM​	​
0.65​	6th / final gear​	​
3.91​	differential ratio​	​
40.1​	10R22.5 tire diameter​	​
3.1415926536​	Pi​	​
125.977865409​	inches per revolution​	= Diameter * Pi​
2384.6153846154​	Xmsn output rpm​	=Eng RPM / Final Gear​
609.8760574464​	axle rpm​	= Xmsn output / Differntial Ratio​
76830.8838811228​	inches per minute​	= Tire Diameter in inches * Axle RPM​
6402.5736567602​	feet per minute​	= Inches per min / 12​
1.2126086471​	miles per minute​	= Feet per min / 5380​
72.7565188268​	mph​	= Miles per min* 60​

If it helps ( I hope ) there is an example of speed calculation by transmission final gear ratio, differential gearing, and RPM and tire size (from manufacturer).

These websites were useful.

https://www.tritrack.net/horsePower.html

https://www.blocklayer.com/rpm-gear

I calculated the HP needed from my probable (wild ass guess) drag coefficient and weight for the speed I desired.

I compared that HP to the chart of HP and torque vs RPM from the engine manufacturer.

My goal was for the lowest RPM still generating about 120%+ of the HP I needed, to have an RPM and gearing still making the highway mph I desired. I assumed about 95~100 sq ft for my roof raised skoolie.

Hope is that on the level and towing, running at ~83%-90% throttle at minimum RPM the exhaust gas temp isn't creeping up past 1200 or so. If it does I gear down

I'm running a 330HP Cat 3126B with an MD3060 and a 3.91 differential.

​

See all those decimal points in the calculations above? Those keep things accurate....but the fact that torque convertor slippage is not known, calculated, or even guessed at makes all those decimal points useless.