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Old 10-26-2020, 07:50 PM   #1
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Join Date: Jul 2020
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Axle Ratio

Hello,

I recently purchased a 2003 Thomas FS65 on a Freightliner chassis. It has a cummins 5.9 engine and an Allison 2000 series transmission with 5th gear being overdrive.
My sticker on the bus says the gear ratio is 5.38. The tires are 10r 22.5.
I cant quite put it together cause when I'm driving down the highway, my rpms are at 2000 while doing 60-63mph.
Other than ripping out the axle is there some kind of math I could do to figure it out?
Thanks for the help anyone. It's just out of curiosity.
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Old 10-26-2020, 09:30 PM   #2
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Join Date: May 2014
Location: West Ohio
Posts: 1,968
Year: 1984
Coachwork: Bluebird
Chassis: International 1753
Engine: 6.9 International
Rated Cap: 65
What can't you put together?

10r22.5 tires will have an average height of 40.2 inches, an allison 2000 has a 5th gear ratio of .71, your rear axle is a 5.38, and your rpm is 2000.

Put all that data into any one of a hundred online speed to rpm calculators, and you get 2000 rpm = 62.64 mph, which is what you're seeing.

What are you trying to figure out? If it's the rear axle ratio, you could always pull the pumpkin and count the teeth to be 100% certain. But based off of your data, I'm thinking the 5.38 sounds correct.
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Old 10-27-2020, 12:57 AM   #3
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Location: Georgia
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Year: 2001
Coachwork: Blue Bird
Chassis: IH
Engine: T444E
Rated Cap: 14
I had a 5.38 in my bus, with same size tires. However, I have a 5 speed manual with a 1:1 top gear and I was seeing 2600 RPM at 55 MPH - the overdrive makes a big difference for you. I swapped my ratio to 3.91 and my top speed is now around 78 MPH, though on the freeway I'm averaging around 70 for top speed on any uphill.


I haven't double checked Booyah's math but based on my own calculations and experience, everything sounds about right. The 5.38 is a fine gear ratio for those who have overdrive gears. Not so much for those with 1:1 top gears.
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Old 10-27-2020, 10:23 AM   #4
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Quote:
Originally Posted by Brad_SwiftFur View Post
I had a 5.38 in my bus, with same size tires. However, I have a 5 speed manual with a 1:1 top gear and I was seeing 2600 RPM at 55 MPH - the overdrive makes a big difference for you. I swapped my ratio to 3.91 and my top speed is now around 78 MPH, though on the freeway I'm averaging around 70 for top speed on any uphill.


I haven't double checked Booyah's math but based on my own calculations and experience, everything sounds about right. The 5.38 is a fine gear ratio for those who have overdrive gears. Not so much for those with 1:1 top gears.
Brad_SwiftFur,

Thanks for your reply. I'm new to all of this and not much of a computer guy. I just figured that the axle was swapped out at some point.

Take care!
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Old 10-27-2020, 12:39 PM   #5
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Join Date: May 2009
Location: Columbus Ohio
Posts: 14,144
Year: 1991
Coachwork: Carpenter
Chassis: International 3800
Engine: DTA360 / MT643
Rated Cap: 7 Row Handicap
0.74 for 5th gear on the common school bus allison (2500 PTS)
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Old 10-27-2020, 12:58 PM   #6
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Join Date: May 2014
Location: West Ohio
Posts: 1,968
Year: 1984
Coachwork: Bluebird
Chassis: International 1753
Engine: 6.9 International
Rated Cap: 65
I've seen the 2000 used on the fs65.
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Old 10-27-2020, 02:39 PM   #7
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Join Date: May 2009
Location: Columbus Ohio
Posts: 14,144
Year: 1991
Coachwork: Carpenter
Chassis: International 3800
Engine: DTA360 / MT643
Rated Cap: 7 Row Handicap
Quote:
Originally Posted by Booyah45828 View Post
I've seen the 2000 used on the fs65.



Columbus city schools still has a few... FS65's with 2500 PTS
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