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Old 07-13-2016, 06:16 PM   #21
Bus Geek
 
Join Date: May 2009
Location: Columbus Ohio
Posts: 7,955
Year: 1991
Coachwork: Carpenter
Chassis: International S3800
Engine: DT360
Rated Cap: 7 Row Handicap
if you have an open diff and are only jacking upo omne wheel doesnt the spider gear spin and double the driveshaft turns?

-Christopher
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Old 07-14-2016, 09:49 AM   #22
Bus Nut
 
Join Date: Apr 2015
Posts: 201
Quote:
Originally Posted by Rider View Post
This is what I've found by vin -

Trans Order Code MT1
Trans Type 4 4.0 min 4.0 max
Trans Description Cont. HD Automatic w/OD
First Gear Ratio (:1) 2.48 2.48 min 2.48 max
Second Gear Ratio (:1) 1.48 1.48 min 1.48 max
Third Gear Ratio (:1) 1.00 1.0 min 1.0 max
Fourth Gear Ratio (:1) 0.75 0.75 min 0.75 max

So, in this case I'd be concerned with the 4th gear ratio?
IF you are trying to determine if the 4th gear is an over drive gear, then yes that would be the number you want to look at.

And, yes, that 4th gear ratio is an overdrive ratio.

I tried to find a laymans explanation ... I think this web site may explain things about over drive ratios for you:

Overdrive - How Automatic Transmissions Work | HowStuffWorks


BUT ... if you want to know about speed and fuel economy, you need to consider the rear axle ratio.
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Old 07-14-2016, 09:59 AM   #23
Bus Geek
 
Join Date: May 2009
Location: Columbus Ohio
Posts: 7,955
Year: 1991
Coachwork: Carpenter
Chassis: International S3800
Engine: DT360
Rated Cap: 7 Row Handicap
all the gears in the workld mean nothing unless you have an engine with enough torque / Horsepower to turn them...

the top gear ratio being NUMERICALLY lower (.75:1) just means your RPM will be lower and that in the top gear the engine has to work harder to turn the wheels..

if your engine has lots of power then having low RPM at highway speed is a good thing... if you are climbing hills foot to the floor and the bus is running against its redline then chances are an overdrive transmission will help.. if you are climbing hills ands your bus is slowing down RPM and speed then overdrive wont help...

-Christopher
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Old 07-14-2016, 10:34 AM   #24
Bus Nut
 
Join Date: Apr 2015
Posts: 201
Quote:
Originally Posted by Rider View Post
This is what I've found by vin -

REAR Axle Ratio (:1) - Rear 4.10, 4.56, 3.73 3.73 min 4.56 max
I believe that the rear axle ratios you've quoted are all that were installed in these buses, but that list does not include any of the posi-traction or locked rear options that were used.

Of the three ratio's, 3.73 would be your best bet for cruising speed and fuel economy.

I've read that if you take the VIN number to a GMC dealership parts department, they can get a build out sheet which will tell you the rear axle that was installed in your bus.

If you decide to do this I would suggest that you go to the dealership yourself and do not call!!! Most dealerships will not do this over the phone. Also, do not go at or near lunch time or closing time.
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