How to determine rear axle ratio's?

[68chevycst]

I just picked up a 72 C50 for parts and I was wondering the best way to determine what ratio's the 2 speed rear axle has. Some one has already removed the glove box door so I don't have a SPID to look at. It doesn't have a motor so I can't take it for a drive either. I'm sure it has some type of identification number on the axle. Is there a way to look up/decode a number?

Thanks,
Seumas

[jumpsoffrock]

I am not aware of any ID numbers that would state the ratio. At this point, jack it up and turn the tires, do some math. Most common were the
J10 : 15000-LB 2 SPEED AXLE 6.5/8.58 RATIO

[rkn463]

Hi Seumas, I agree with jumpsoffrock. I expect you already know this but you can lift just one side and rotate that wheel two revolutions and count the number of times the drive shaft goes around. Additionally if if it is a vacuum operated 2 speed then it is easy to check hi and lo ratio by applying vacuum and checking it, then release vacuum and check again. I don't recall whether default (no vacuum) is hi or lo..

You said you bought it for parts, make sure to let us know what is left over before you scrap it out!

-Richard

[BlueDiver]

My '60 C50 has a single speed rear end and maxes out at a whopping 32 MPH at the moment. I am trying to figure out its gear ratio so I can search out a different rear end or replacement gears to get interstate speeds.

It has a 236 straight six in it now /Sm240 trans with 4 spd man but I will be replacing it with a 5.9L Cummins 6bt with 5 spd man.

I'm interested in any help as well.

[jumpsoffrock]

Originally Posted by BlueDiver

My '60 C50 has a single speed rear end and maxes out at a whopping 32 MPH at the moment. I am trying to figure out its gear ratio so I can search out a different rear end or replacement gears to get interstate speeds.

It has a 236 straight six in it now /Sm240 trans with 4 spd man but I will be replacing it with a 5.9L Cummins 6bt with 5 spd man.

I'm interested in any help as well.

Most important would be to find what RPM is 32mph. If that is 3000rpm, then you might have something like 8.5:1 gears, which are really low for single speed rears.

[68chevycst]

Thanks guys for the input. I was figuring that I was just going to have to jack it up and do some math. Figured that I would see if there was any numbers to decode first. I will keep you posted on the parts. I should be parting two trucks. I'm hoping that the latest truck has the higher gear ratio since it had a moving box on the back. Here are some pictures.

Seumas

[JPBrecheisen]

Originally Posted by BlueDiver

My '60 C50 has a single speed rear end and maxes out at a whopping 32 MPH at the moment. I am trying to figure out its gear ratio so I can search out a different rear end or replacement gears to get interstate speeds.

It has a 236 straight six in it now /Sm240 trans with 4 spd man but I will be replacing it with a 5.9L Cummins 6bt with 5 spd man.

I'm interested in any help as well.

Chances are, its a 7.20:1 ratio for that year and set up. I have a sales brochure for the big trucks of that year. I can take a look and post back. Mine is a C60 with 261 6cyl SM420 single speed rear (7.20:1) and its slow as hell also.

[BlueDiver]

Originally Posted by jumpsoffrock

Most important would be to find what RPM is 32mph. If that is 3000rpm, then you might have something like 8.5:1 gears, which are really low for single speed rears.

The truck doesn't have a tach but its screaming to the point I don't want push any more for speed. I'm guessing RPM is petty close to 3000.

I have a line on another rear end but I am trying to figure out whether it is cheaper to get that or replace gears in my original rear end.

[BlueDiver]

Originally Posted by JPBrecheisen

Chances are, its a 7.20:1 ratio for that year and set up. I have a sales brochure for the big trucks of that year. I can take a look and post back. Mine is a C60 with 261 6cyl SM420 single speed rear (7.20:1) and its slow as hell also.

That would be great. I found one PDF online that had some info but I couldnt print or download it. I would really like to get some type of manual for it.

[JPBrecheisen]

Originally Posted by BlueDiver

That would be great. I found one PDF online that had some info but I couldnt print or download it. I would really like to get some type of manual for it.

Looks like a C50 with 13k lbs rear single speed was 6.60:1 per the factory brochure for that year.

[BlueDiver]

Nice book. That would be sweet to find one.

I opened up the rear end and found some numbers but I'm not exactly sure what they mean yet. Any clues? :

3758882 GM 5-33 1 60 1 <-- The last 1 is twice as large as the rest of the numbers.

There are some other numbers on the coupling that holds the gears in as well. Those numbers are: 3652275 (the last number may be a 6 or an 8 rather than 5)
8

[JPBrecheisen]

Originally Posted by BlueDiver

Nice book. That would be sweet to find one.

I opened up the rear end and found some numbers but I'm not exactly sure what they mean yet. Any clues? :

3758882 GM 5-33 1 60 1 <-- The last 1 is twice as large as the rest of the numbers.

There are some other numbers on the coupling that holds the gears in as well. Those numbers are: 3652275 (the last number may be a 6 or an 8 rather than 5)
8

The 3758882 is a part number for your gear set. The 5-33 is the tooth count on ring and pinion. 5 teeth on pinion, 33 teeth on ring gear. To find the ratio, you divide 33 by 5 and you get 6.60:1

You have the 6.60:1 ratio.

[BlueDiver]

Thanks JP!!! That helps a lot. Now that opens up more questions that I have to figure out. The truck is still maxing out at 32 MPH. Well, I think it is maxed out. Sounds that way. I don't have a tach and I don't have it in me to push the pedal down any more to see what happens. I thought with 6.60:1 it would be somewhere in the 45 range.


I have tried to reference the part number but have had some trouble thus far.

[JPBrecheisen]

Originally Posted by 68chevycst

I just picked up a 72 C50 for parts and I was wondering the best way to determine what ratio's the 2 speed rear axle has. Some one has already removed the glove box door so I don't have a SPID to look at. It doesn't have a motor so I can't take it for a drive either. I'm sure it has some type of identification number on the axle. Is there a way to look up/decode a number?

Thanks,
Seumas

Try to send any numbers you can find like bluedriver. I have a parts book I can take a look at and hopefully figure out what you have.

[JPBrecheisen]

Originally Posted by BlueDiver

Thanks JP!!! That helps a lot. Now that opens up more questions that I have to figure out. The truck is still maxing out at 32 MPH. Well, I think it is maxed out. Sounds that way. I don't have a tach and I don't have it in me to push the pedal down any more to see what happens. I thought with 6.60:1 it would be somewhere in the 45 range.


I have tried to reference the part number but have had some trouble thus far.

I am sure the truck can do 45. Remember they were made to move a lot of weight with a small engine, so the engine is gonna run at highway speeds. My C60 is the same way. If you're curious, wire a temporary tach into it and see where it's at. I suspect you're gonna be around 3k+ at 45-50mph. The 235/261s get pretty noisy at those rpms, but most seem to say it won't hurt em. Only thing I would mind is valve seats, especially on exhaust side.

[rkn463]

Originally Posted by BlueDiver

Thanks JP!!! That helps a lot. Now that opens up more questions that I have to figure out. The truck is still maxing out at 32 MPH. Well, I think it is maxed out. Sounds that way. I don't have a tach and I don't have it in me to push the pedal down any more to see what happens. I thought with 6.60:1 it would be somewhere in the 45 range.


I have tried to reference the part number but have had some trouble thus far.

Measure the tire height and plug the numbers in to an RPM calculator like this one: https://www.ringpinion.com/calculators/Calc_RPM.aspx

[BlueDiver]

Cool, thanks!

[Softpatch]

Great thread....Sp
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