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Old 03-07-2016, 05:26 PM   #1
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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, 06:04 PM   #2
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Depends whats in it. 80 or 75/90w or synthetic.
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Old 03-07-2016, 06:05 PM   #3
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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, 06:10 PM   #4
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Wow, I need to learn to read slower. [sigh]
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Old 03-07-2016, 07:15 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 03-07-2016, 11:17 PM   #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 03-07-2016, 11:20 PM   #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 03-07-2016, 11:53 PM   #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, 06:35 AM   #9
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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, 07:33 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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Old 03-08-2016, 07:54 AM   #11
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Beyond achieving the desired ratio, there are a number of other considerations when swapping rear axles.

Weight rating:
It has to at least match the recipient vehicles GVAW

Overall Width: It's gotta fit (pretty obvious)

Width of spring perches:
)a little less obvious, but important) they can be relocated but it's not all that easy ( I know...I had to move mine)

Shock or air bag location:
(if it has any) here again they can be relocated but will involve more work, time & $$$

Tire/Wheel size:
Ideally the same as what is on your vehicle (otherwise, you change them all)

Lug bolt pattern: Here again, unless the wheels on the donor match your existing patterns, figure on spending mo money to get them all the same or having different wheels front and rear ( which spare to carry?...room for two?)

These are just a few of the things I recall having to deal with while searching for highway gearing on my old rig. Of course if you can find the ratio you need as just a ring & pinion set...the other issues take care of themselves.

Best of luck and anyone else with thoughts on axle swaps...please chime in.
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Old 03-08-2016, 08:20 AM   #12
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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?
Basically what Tango mentions. It most certainly can be done, but unless the current original axle has issues other than ratio, the only real reason to change out housings would be personal preference for some reason. Gears are pretty easy. You can change out the gear head with no real special tools, or the gear set, but would require some special tools and knowledge.

Changing out the whole axle housing, can be done, but requires a lot of measuring and fitting.
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Old 03-08-2016, 10:17 AM   #13
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Changing out the gears is easy peasy with a motorcycle jack to help pop it in there. Our bus started with miserable 6.11 ratio and went 47mph tops. Got a 4.78 for a good price from a guy on skoolie, now we get about 60mph.
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Old 03-08-2016, 10:33 AM   #14
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I did a check - a '95 MCI had a 4.11 - they are a highway bus...but it didn't show the gearing in the transmission.
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Old 03-08-2016, 10:36 AM   #15
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One of the very few questions I asked before bidding on my bus was what the rear axle ratio was. They told me 4.44 and I got excited! I get about 63mph out of a forty footer with 195 hp. Does pretty well in the mountains, too.
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Old 03-08-2016, 12:34 PM   #16
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Now there's an issue that I hadn't considered before. Are the rear axles on a rear engine bus of the same basic configuration as that of a conventional (front engine and driveshaft) setup? Come to think of it, does a rear engine bus have a driveshaft? Here I'd been assuming that it was an integrated setup like a front engine front wheel drive car, and I have no reason to make that assumption. (You know what happens....)

Looking at it from the outside as I am now, and with the usual amount of teenaged screwing around with cars under my belt, I would be more scared of swapping axles than raising a roof. The only thing I'd consider would be a new-gearset-same-punkin situation.
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Old 03-08-2016, 12:58 PM   #17
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Buses with rear engines have a driveshaft. They can be very short with short being defined as less than 36" which includes a slip joint.

The differentials on rear engine buses are generally identical to any other differential except they are mounted in a mirror image so that a right turning engine will turn the wheels in the correct direction. If it isn't installed mirror image you will have one forward gear and lots of gears in reverse.

The advantages of swapping a complete assembly is you can upgrade to better brakes. Most Type 'D' RE buses have 9" rear brakes. But some lighter duty buses have as small as 7" rear brakes while some heavy duty buses have as large as 13" rear brakes.

The nice thing about newer buses that have been built in the last few years is that an IC bus uses standard IHC medium duty truck parts, a Thomas bus uses standard Freightliner medium duty truck parts, and a Blue Bird uses standard Volvo medium duty truck parts. Going to a junk yard and finding something that will be a bolt in swap is relatively easy. A Rockwell Meritor axle under a bus with 10-hole hub piloted wheels is basically the same thing as a Rockwell Meritor axle under a truck with 10-hole hub piloted wheels. For that matter you could upgrade to an air suspension if you don't have one already.

You may actually discover it is easier to swap a whole assembly than trying to swap out the gears in the pumpkin.
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Old 03-08-2016, 01:07 PM   #18
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Yeah I can see the mirror image thing; the driveshaft attaches to the diff at the rear, not the front. Thanks for the answer! I won't be so scared of it anymore. I'm sure I could figure out backfitting the pneumatics for air ride; would I have to weld fittings to the frame?

And, by "newer buses that have been built in the last few years" do you mean 15 year old buses that come up for auction? Or newer than that? Thanks in advance -
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Old 03-08-2016, 02:09 PM   #19
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I have seen MCI's cruising at 80 mph, and per drivers, they get 9-11 mpg!
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Old 03-08-2016, 10:35 PM   #20
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I did a check - a '95 MCI had a 4.11 - they are a highway bus...but it didn't show the gearing in the transmission.
Find out what transmission make and model and google it. If it's an MCI with an automatic, it'll be an allison more than likely. Detroit Allison would be a contact to find info.
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