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Old 10-04-2016, 03:28 PM   #1
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Trying to identify my rear axle ratio, help?

Hello everyone, I just purchased a 1990 Chevrolet shorty with a Detroit Diesel 8.2. I am trying to identify my rear axle ratio so i can calculate if my motor's governor has been turned down. I've crawled all under and around the back end of the bus, but the stamped ID tag has been coated with bus undercoating and is unreadable. I have copied all of the casting numbers off the rear end, but i don't know if that will help identify the axle. It has some weird bolt with a set nut sticking out of the drivers side of the chunk so i my ridiculously hopeful side It may be a 2 speed axle and its just set to the lowest ratio and the switch gear was never hooked up. I doubt it though. So... if there is anyone that thinks they can identify the axle ratio based on the casting numbers here we go...

Eaton Axle:
210316e with a NF and a DI above it, this is on the third member on the drivers side front.

210339 , this is up by the input shaft, where the driveshaft hooks to the universal joint.

A210340 Not real sure where this one was

01300 Again, not sure where this one is located

and on the only readable riveted plate is stamped with nothing else readable:

PT NO 05989 (maybe)
HSG Model
HSG Cap
HSG ID NO


Good Luck:
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Old 10-04-2016, 06:26 PM   #2
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Look like this?


First hit when googling "1990 chevrolet school bus eaton axle":

GMC School Bus REAR AXLE/EATON
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Old 10-04-2016, 06:42 PM   #3
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No, thats not it. Where that flat plate thing is on the 3rd member there is a large bolt, like 1" in diameter with what looks like a locking nut that sticks out perpendicular to the driveshaft (parallel to the axle tube) this nut is hollow, it may have some actuator or connectors inside of it but its all full of goo so i don't really know.
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Old 10-04-2016, 06:45 PM   #4
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On my 86 eaton rear data plate with the pt.no and msg model..
The numbers are stamped into the plate so maybe you can pull it of and read it from the back side?
The other data plate which tells the model,and the gear ratio is in line with the drive shaft on top. Not quite on the very top but in between the drive shaft and the axle housing. And those numbers are raised lettering so if you can find it maybe you can pull it and do the same
I will try to look at my casting numbers when I get a chance and see if anything matches.
Sorry I can't help with the governor adjustments? Maybe if the weather not bad I can go see an old diesel mechanic friend of mine and ask? I have been trying to find time to take my bus to see him anyway.
Good luck
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Old 10-05-2016, 10:45 AM   #5
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Link posted will take you to more information.

If it looks almost like the picture above, sounds like you have a 2 speed axle minus the actuator, I don't know that for sure, but sure seems like it.
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Old 01-17-2017, 09:42 AM   #6
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According to my buses build sheet I have a chev 350, an Allison at545, Rear axle is 7.20:1, tires are 10 R20.5 How fast is it running @ 4000 rpm?

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Old 01-17-2017, 09:51 AM   #7
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im going on the assumption of a standard truck tire height of 41". and youd be able to do close to 65 .. your At545. may limit that if your redline is 4000 as it's converter will slip and allow the revs to go up under load..

if I knew the exact tire height we can calculate it easier...

https://www.ringpinion.com/calculators/Calc_RPM.aspx

-Christopher
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Old 01-17-2017, 10:08 AM   #8
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what if the R&P or entire center section has been replaced ?
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Old 01-17-2017, 10:24 AM   #9
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Originally Posted by cadillackid View Post
im going on the assumption of a standard truck tire height of 41". and youd be able to do close to 65 .. your At545. may limit that if your redline is 4000 as it's converter will slip and allow the revs to go up under load..

if I knew the exact tire height we can calculate it easier...

https://www.ringpinion.com/calculators/Calc_RPM.aspx

-Christopher
My mistake my tires are 8.25 R 20 and roughly 37" diameter. I was not home when I asked.

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I'm worried that I might have done damage driving home last fall. Driving some mountain down hills at over 65 mph.
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Old 07-15-2017, 05:42 PM   #10
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Axle ratios are relatively easy to figure out. Jack up one wheel and chock the rest and put the tranny in neutral. Make a mark on the bottom of the tread and have someone count the drive line revolutions as you rotate the tire one exact turn and tell us what you find. The "bolt" that is sticking out of the side of the drop out is there to alleviate pinion deflection under load. DON'T mess with it, there is a very specific distance that it has to be from the pinion shaft.
Rob
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Old 07-15-2017, 05:54 PM   #11
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I thought only one wheel would double the revolutions of the driveshaft due to the spider gears spinning?
-Christopher
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Old 07-15-2017, 05:56 PM   #12
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Originally Posted by Tootalltechie View Post
My mistake my tires are 8.25 R 20 and roughly 37" diameter. I was not home when I asked.

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I'm worried that I might have done damage driving home last fall. Driving some mountain down hills at over 65 mph.
with a gas notor as long as you didnt over-rev it or foam the oil / get it hot it should take it.. I dont know much about running on propane to know if it causes hotter cylinder temps etc...

ive built hotrods with 350s in the past that I was taching 3000+ on the highway (they were holeshot cars more than cruisers)..

-Christopher
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Old 07-16-2017, 10:47 AM   #13
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Originally Posted by cadillackid View Post
I thought only one wheel would double the revolutions of the driveshaft due to the spider gears spinning?
-Christopher
Nope, it doesn't work like that, there is no multiplication from the spider gears.
Rob
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Old 07-16-2017, 12:30 PM   #14
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Quote:
Originally Posted by cadillackid View Post
I thought only one wheel would double the revolutions of the driveshaft due to the spider gears spinning?
-Christopher
Quote:
Originally Posted by White Lightning View Post
Nope, it doesn't work like that, there is no multiplication from the spider gears.
Rob
Actually there is some truth to this. The gear ratio comes into play here. When you are going down the road and all wheels are spinning the same speed, you'll get 1 tire revolution for every X driveshaft revolutions. So far, so good, right?

Now, when you go around a corner, the outer tire needs to spin faster than the inner tire. That's the entire reason we have differentials in the first place. For our discussion, it doesn't matter how much difference there is, you still end up with the same amount of tire rotation with the same amount of driveshaft rotation; normally it's split evenly between the tires but around a corner, one gets more, the other gets less.

Following this logic, if we stop one wheel completely (and we have an open differential, not a locking or limited slip one), the differential still does it's job, but now you do indeed get double the tire revolutions for the same number of driveshaft revolutions. Working in reverse (to calculate gear ratio), remember you're getting half the driveshaft revolutions since you're only turning one wheel. Either rotate the tire exactly twice, or double how much the driveshaft turns to get a correct result.
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Old 07-16-2017, 01:01 PM   #15
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Originally Posted by Brad_SwiftFur View Post
Actually there is some truth to this. The gear ratio comes into play here. When you are going down the road and all wheels are spinning the same speed, you'll get 1 tire revolution for every X driveshaft revolutions. So far, so good, right?

Now, when you go around a corner, the outer tire needs to spin faster than the inner tire. That's the entire reason we have differentials in the first place. For our discussion, it doesn't matter how much difference there is, you still end up with the same amount of tire rotation with the same amount of driveshaft rotation; normally it's split evenly between the tires but around a corner, one gets more, the other gets less.

Following this logic, if we stop one wheel completely (and we have an open differential, not a locking or limited slip one), the differential still does it's job, but now you do indeed get double the tire revolutions for the same number of driveshaft revolutions. Working in reverse (to calculate gear ratio), remember you're getting half the driveshaft revolutions since you're only turning one wheel. Either rotate the tire exactly twice, or double how much the driveshaft turns to get a correct result.
oops yep I had that backwards.. double the tire revs is correct..
-Christopher
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Old 07-17-2017, 01:16 AM   #16
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Gentlemen, again, there is no multiplication through the spider gears. If you jack up one wheel, put the drive line in neutral, spin one tire, hold the other in place, and count the drive line revolutions you will get the final drive ratio. Guaranteed lol. I've done it hundreds of times in order to ball park the ratio. All you need from there is a list of the available ratios and you have the numbers you need, along with the tire diameter to figure drive line RPM at any given MPH. Most non lock up torque converters that are factory stall speeds will give you right about 200 RPM above the drive line RPM for a crankshaft RPM. Give it a try.....
Rob
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Old 07-17-2017, 01:51 AM   #17
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Gentlemen, again, there is no multiplication through the spider gears. If you jack up one wheel, put the drive line in neutral, spin one tire, hold the other in place, and count the drive line revolutions you will get the final drive ratio. Guaranteed lol. I've done it hundreds of times in order to ball park the ratio. All you need from there is a list of the available ratios and you have the numbers you need, along with the tire diameter to figure drive line RPM at any given MPH. Most non lock up torque converters that are factory stall speeds will give you right about 200 RPM above the drive line RPM for a crankshaft RPM. Give it a try.....
Rob
Rob, you are mistaken. If both sides are turning, then yes, this is true. However, if one is stationary then the other gets the difference (hence the term "differential"). Hot-rodders can even tell you if you jack up one wheel, start the car, put it in drive and floor the throttle, you can easily spin that one tire well over it's speed rating.
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Old 07-17-2017, 02:02 AM   #18
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The spider gears do not "multiply" anything. Their purpose is to simply make the differential work. When you're going straight down the road, the spider gears are not turning relative to each other (they are stationary in the housing, going along for the ride). When you take a corner, they turn so as to allow the wheels to rotate at different speed but overall the vehicle is still moving forward at X MPH. Again, no multiplication, just allowing a difference between the sides. The tighter the turn, the greater the difference.

Farm tractors often have independent wheel brake which allow the driver to slow or stop one drive wheel; the intent to allow the outer wheel to help turn the tractor and give a near-zero-turning-radius. Assuming the differential is open (many have diff locks), the same applies. The input shaft still turns at the same speed but the outer wheel spins faster as it is receiving the difference from the non-turning wheel. Still no multiplication.
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Old 07-17-2017, 10:37 AM   #19
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Thank you Brad.
Rob
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Old 07-17-2017, 12:08 PM   #20
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this is exactly what I have experienced.. often if you jack up both wheels on an open dif, the spider gears will turn instead of the ring gear... turning one wheel results in the other turning backwards..

-Christopher
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