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Old 03-07-2016, 06:26 PM   #1
Skoolie
 
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differentials

What differentials/gearing is used on most fe 35-40ft d busses?
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Old 03-07-2016, 07:04 PM   #2
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Depends whats in it. 80 or 75/90w or synthetic.
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Old 03-07-2016, 07:05 PM   #3
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Year: 1946
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They are all over the board. Skoolies rend to be a bit lower while transits are more often highway geared. Even then, the actual ratios are dependent on the engine and transmission specs. That said, it is definitely something one needs to know before purchasing any given unit to avoid some serious expense.

My old 1946 Chevy was built before there was even a highway system ,so top speed on it was about 45 mph. It now has a new rear axle that will allow for 65-70 but I was anticipating that expense.
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Old 03-07-2016, 07:10 PM   #4
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Wow, I need to learn to read slower. [sigh]
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Old 03-07-2016, 08:15 PM   #5
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Often in the high 4s to mid 5s. 4.10:1 is a common axle ratio for vocational use. But they can go up into the 5s. But it will depend a lot if the bus is gasoline or diesel powered. Diesel powered buses would be lower numerical final ratios than a gasser on average.
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Old 03-08-2016, 12:17 AM   #6
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Thanks. So a diffy that is 3.x is likely to provide better mpg on the freeway but slower acceleration?
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Old 03-08-2016, 12:20 AM   #7
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If you want to change the differential what kinds of vehicles would you swap from? 1 ton pickups? Deuce and a half? Garbage trucks?
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Old 03-08-2016, 12:53 AM   #8
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try a gear calculator like this one:

Tire Size, RPM, Speed, and Differential Ratio Calculator

my bus will do 2500 rpm, has 42.5" tire diameter, and a 4.7 rear differential.

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Old 03-08-2016, 07:35 AM   #9
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Quote:
Originally Posted by pengyou View Post
Thanks. So a diffy that is 3.x is likely to provide better mpg on the freeway but slower acceleration?
In general, that is what you would experience. But if you go thru any hill country, it will also make a difference.

The school bus was only made for local, short distance runs with a million stop and go's. So high numerical ratios make sense. If you wish to go long distance, as in rv usage, then yes, you are on the right track to find the best ratio for your intended usage.

There are various ways to find your current ratio. Some may methods may even be on youtube now days. But when all else fails, you can get a friend to help and jack up one side of the rear axle on a level surface, chock well the other wheels and put a heavy duty jack stand under the axle for safety. Always remember safety is your top priority.
Then with a piece of chalk, mark a spot on the tire that you can index pretty accurately with something on the buss exterior. Mark the drive shaft the say way. While the friend rotates the wheel two full revolutions very slowly, you count the drive shaft revolutions. Then divide the drive shaft count by 4 and you'll have your ratio.
An example would be 2 tire revs and just a tick under 16.5 on the drive shaft would be a 4.10:1 ratio.

You'll have to find the axle brand and model to be able to find a replacement gear head or gear set. Likely your bus would have a heavy truck axle under it. Likely a mainstream brand like eaton or rockwell which is now known as merito. Could be as low of rating as 20,000lbs, but should be a 23,000lbs rated on a the full size buses. Just depends how old it is and what the requirements were at that time.
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Old 03-08-2016, 08:33 AM   #10
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How hard would it be to get an axle from an over the road truck from a salvage yard and swap it for the stock axle? Are there any huge hurdles that would have to be overcome or that would prevent this from being done?
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