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Old 03-22-2024, 01:14 PM   #1
Mini-Skoolie
 
Join Date: Aug 2023
Location: San Diego, CA
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Coachwork: SS-17 3700
Chassis: 3700 short bus
Engine: IHC 7.3 A170
Rated Cap: 21500
Recommendations for rear end gear ration change

Hello All, I have a 1993 International 3700 24' school bus that has a 7.3 L IDI engine

The bus weighs 13,000 lbs and had a GVWR of 21500.

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

I would like to change the carrier assembly to one that is better suited for freeway driving.

Has anyone updated their gearing and what ratio would be best without affecting performance too much.

Thanks

Joe

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Old 03-22-2024, 01:19 PM   #2
Bus Geek
 
Join Date: May 2009
Location: Columbus Ohio
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Year: 1991
Coachwork: Carpenter
Chassis: International 3800
Engine: DTA360 / MT643
Rated Cap: 7 Row Handicap
the IDI is a pretty low horsepower engine... im not sure what tires you have (size) so its hard to recommend.. but id probably plan on keeping it 55-60 MPH or so. and if running 11R22.5 tires, use a 4.78 ratio.. a 4.44 would give you better highway speed but on inclines you'll slow down...



again really hard to put out actual ratios without knoeing tire size..
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Old 03-22-2024, 01:40 PM   #3
Mini-Skoolie
 
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Coachwork: SS-17 3700
Chassis: 3700 short bus
Engine: IHC 7.3 A170
Rated Cap: 21500
@cadillackid ,Thanks for the quick reply I have 9R 22.5 tires currently

Joe
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Old 03-27-2024, 11:20 AM   #4
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Chassis: International 1753
Engine: 6.9 International
Rated Cap: 65
5.57 with a 10r22.5 tire and an at545 will get you 70 mph. I know this because I have that engine with these specs on my bus, and that's what it will do. However, any hill, and you're slowing down. An NA IDI just doesn't have the HP to do it. As christopher said, you won't have enough power to go faster, and you can't add more power without a turbo.


So don't change the rear gear unless you add a turbo, intercooler, and larger injection pump. Likely need a new trans as well, unless you've got a manual. When I was dreaming about upgrading my bus with a turbo, intercooler, injection pump, and the mt643, I was looking for a 4.78 in a 2 speed axle. With my 3500 rpm governed speed, that would have given me a max of 65 mph in low range with plenty of acceleration, and then a max of 87 mph in high range. Plan was to keep it in low whenever not on the interstate, and then shift it into high range once on the interstate.

My bus is for sale, and those dreams have gone the way of the wind.
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Old 03-27-2024, 11:26 AM   #5
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Join Date: May 2009
Location: Columbus Ohio
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Chassis: International 3800
Engine: DTA360 / MT643
Rated Cap: 7 Row Handicap
Quote:
Originally Posted by Booyah45828 View Post
5.57 with a 10r22.5 tire and an at545 will get you 70 mph. I know this because I have that engine with these specs on my bus, and that's what it will do. However, any hill, and you're slowing down. An NA IDI just doesn't have the HP to do it. As christopher said, you won't have enough power to go faster, and you can't add more power without a turbo.


So don't change the rear gear unless you add a turbo, intercooler, and larger injection pump. Likely need a new trans as well, unless you've got a manual. When I was dreaming about upgrading my bus with a turbo, intercooler, injection pump, and the mt643, I was looking for a 4.78 in a 2 speed axle. With my 3500 rpm governed speed, that would have given me a max of 65 mph in low range with plenty of acceleration, and then a max of 87 mph in high range. Plan was to keep it in low whenever not on the interstate, and then shift it into high range once on the interstate.

My bus is for sale, and those dreams have gone the way of the wind.

4.78 high range always seems hard to find too.. never understood why so many 2 speeds still topped out in the 5.13 or 5.39 high range..



I always thought a 2 speed with a 5.57 low and a 4.44 high coupled to a 5 speed manual (top gear 1:1) and 112R22.5 tires would be a great setup .. you'd have essentially 7 regular gears with a couple being redundant.. of course ai see that fro mthe perspective of an I6 diesel with a max RPM of 2600-2800
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Old 03-27-2024, 11:42 AM   #6
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Quote:
Originally Posted by cadillackid View Post
4.78 high range always seems hard to find too.. never understood why so many 2 speeds still topped out in the 5.13 or 5.39 high range..



I always thought a 2 speed with a 5.57 low and a 4.44 high coupled to a 5 speed manual (top gear 1:1) and 112R22.5 tires would be a great setup .. you'd have essentially 7 regular gears with a couple being redundant.. of course ai see that fro mthe perspective of an I6 diesel with a max RPM of 2600-2800
10000000000000%

What I wanted seemed to be non-existent. I found 2 speeds in in 40k lb axles in the 3.xx range for semi trucks. And I found 19k lb 2 speed axles in the 5.xx or 6.xx range. But never in the range that I was looking for, and I looked hard.

Last thought I had was just to settle for a single speed 4.78 and add a boatload more power to overcome the "slowness" of being under-geared.

Probably shouldn't have sold the spicer auxiliary trans that I had. I didn't want to build linkages to make it work, and since I found it easily enough, I figured I could find an air shift auxiliary trans or a nice 2 speed axle. The latter options never happened, and I probably should have stuck with it and built the dang linkages.....

The old a bird in hand is worth 2 in the bush analogy strikes again.
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Old 03-27-2024, 12:08 PM   #7
Bus Geek
 
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a friend of mine built a dual stick thomas.. his IDI blew up so he dropped in a DT360.. the AT545 blew up so he manual swapped it.


his primary is a 5 speed spicer and the aux is a 4 speed.. now theres obviously some redundant gears.. but wow! that bus can pull a house on the low end and be a highway cruiser on the high!
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Old 04-06-2024, 07:16 PM   #8
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Quote:
Originally Posted by jpitz31 View Post
Hello All, I have a 1993 International 3700 24' school bus that has a 7.3 L IDI engine

The bus weighs 13,000 lbs and had a GVWR of 21500.

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

I would like to change the carrier assembly to one that is better suited for freeway driving.

Has anyone updated their gearing and what ratio would be best without affecting performance too much.

Thanks

Joe

I've seen Chuck Cassady on YouTube do that with his Dana Spicer rearend. I believe that he went from high 4.to a medium 3.and gained highway speeds with that. But he wasn't using an idi engine. He got a remanufactured rearend gears from LKQ. He also replaced the bearing seals in both axles when he did it.
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Old 04-07-2024, 09:05 AM   #9
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I have the same same 7.3 IDI in an Older 89 Ford tow truck. It weighs 11K empty has an GVW of 15K And only has 32 inch tall tires. It has 5.13 gear and 5 spd. (In 5th gear it makes the overall ratio 410)
It does 70 at 3k,….on flat ground.
5th is useless the moment you hit a hill.
If loaded heavy 4th isn’t all that useful.
A IDI makes max torque at about 1500 rpm
And Max HP at about 3K

I’d add a aux trans that could be used on flat ground.
So that you could go back to factory gearing when you hit hills.
I would Not swap rear gears.
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Old 04-07-2024, 10:27 AM   #10
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Engine: T444E, Allison 2000
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Quote:
Originally Posted by Ourvanlives View Post
I've seen Chuck Cassady on YouTube do that with his Dana Spicer rearend. I believe that he went from high 4.to a medium 3.and gained highway speeds with that. But he wasn't using an idi engine. He got a remanufactured rearend gears from LKQ. He also replaced the bearing seals in both axles when he did it.
I've seen that video and made my roll my eyes. He over geared that bus to the extreme with a at545. That trans. is going to pay for it, strain an already anaemic trans on hills, dumps a boat load of heat into the system. I recall he was running a stock dt466 engine but I would be concerned of EGT's on a hard pull up a steep grade with high engine and trans. temps.

I know he was looking at a trans. upgrade if his failed him and his bus was smaller in size but what he did, I simply would never recommend with the ratio he put in.

Lugging an engine does not increase fuel efficiency. Times you will need to do 55mph and things will strain. I would rather over rev an engine a couple 100 rpm than under rev an engine a couple 100 rpm from the ideal sweet spot while highway cruising.
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