[choppy71]

I have a 71 chevy truck rear end, and I am trying to identify what ratio is in it using the number to the left facing the rear end. It says C 25 1 and this does not make sence to me. I thought it was a two digit # like HC. Any ideals would be appreciated.

I am almost sure it is a 373 gear ratio what they call second edition 8.875 rear end but I am not sure.

[Henry Highrise]

Welcome, place your question in the transmission/rearend forum and it will get more response.

[poncho62]

Moving to trans/rear

[66GMC]

Originally Posted by choppy71

I have a 71 chevy truck rear end, and I am trying to identify what ratio is in it using the number to the left facing the rear end. It says C 25 1 and this does not make sence to me. I thought it was a two digit # like HC. Any ideals would be appreciated.

I am almost sure it is a 373 gear ratio what they call second edition 8.875 rear end but I am not sure.

You're probably right in assuming that.

Here are a couple of ways to determine the ratio (at least approximately) if nobody can decode that for you:

1.) Remove the cover ... count the number of teeth on the crown gear, and divide by the number on the pinion. (My 66 GMC 8.875" is 41/11 = 3.73)

2.) Assuming a non-posi diff ... Take a tire crayon and make a mark on the tire and another on the pinion flange. Put the transmission in neutral, block the front wheels, then jack up one wheel (the one with the tire crayon mark on it <grin>). Get under the truck, and turn the driveshaft ... counting how many driveshaft revolutions it takes to get one rotation of the tire.

HTH,
Don

[jimfulco]

If you remove the cover, look on the edge of the ring gear, and you might find some numbers stamped into it. One of them is the tooth count for the ring gear (usually between 35 & 45), and one is the count for the pinion gear (usually between 8 & 16). Other numbers you might see will be a date code.

For example, if you see "41 11 12 70", you'll have a ratio of 41/11 (3.73), and it was made in December 1970.

[choppy71]

I am still wondering if I can identify my rear end by the number to the left of the cover ( C 25 1 ) this is odd because I thought is was a two digit lettering. I know how to do the math when it comes to rotating the wheel or taking off the cover and counting the teeth. What bothers me is the fact that I read books stating that there should be a two digit # that identifys my rear end. The rear end is the original wich came with the truck - 12 bolt second edition 8.875 I THINK. Does the number I mentioned above ( C 25 1 ) mean anything to you guys.

[Triaged]

In trucks that old I never trust any way other then opening up the diff because who is to say that the 5th owner didn't change the gears . Chances are the diff oil needs to be changed anyways.

If you are the 2nd owner and the 1st is someone that told you they didn't change anything on it then I wouldn't worry about getting dirty and would just look up the RPO code in the glove box.

[Blazin72]

That number you are looking at probably wont tell you what gears are inside. Im assuming it must have been out of a half ton pickup? Was it a 4x4 or a 4x2? My 4x4 72 Blazer came with 4.10s (Dana 44 up front still has original tag hanging off the diff cover), my friends 71 3/4 ton 4x4 came with 4.10s too (it has the ancient H072 rear axle though, ever hear of one of those?) I dont know what the 4x2s came with though, seems to me that a there was a general consensus on this board a few days ago that many of them came with 3.08s.

You could try www.67-72chevytrucks.com and see what you can come up with there.

[Triaged]

Originally Posted by Blazin72

That number you are looking at probably wont tell you what gears are inside. Im assuming it must have been out of a half ton pickup? Was it a 4x4 or a 4x2? My 4x4 72 Blazer came with 4.10s (Dana 44 up front still has original tag hanging off the diff cover), my friends 71 3/4 ton 4x4 came with 4.10s too (it has the ancient H072 rear axle though, ever hear of one of those?) I dont know what the 4x2s came with though, seems to me that a there was a general consensus on this board a few days ago that many of them came with 3.08s.

You could try www.67-72chevytrucks.com and see what you can come up with there.

FWIW my 71K5 came with 3.73's in it and so have some of my friends 1st gen blazers.

[Blazin72]

Im not saying his doesnt have 3.73s. Im just saying that rear end could have any number of different ratios in it. I thought mine had 3.73s too when I first bought it. I was pleasantly surprised to see it had 4.10s.

[Triaged]

Originally Posted by Blazin72

Im not saying his doesnt have 3.73s. Im just saying that rear end could have any number of different ratios in it. I thought mine had 3.73s too when I first bought it. I was pleasantly surprised to see it had 4.10s.

That was kind'a the point I was making...wasn't meaning to step on any toes

[Blazin72]

Just checked and it appears all toes are still intact.


Gotta love those old Chevy pickups and Blazers. It's the best body style they've ever had. In my humble opinion of course. Dont care much for the 67s and 68s but the 69-72s look great.

[454C10]

67 to 72 C10's with factory installed v8's and auto transmission normally had 3.08's. 4x4's and 6 cylinder trucks normally had 3.73's and sometimes 4.11's. But a lot can happen to a truck in 35 years.

What kind of rpms is the engine turning going down the highway?

At 70 mph with a 28" tire and a 3.08 gear will run about 2600 rpm. A 3.73 will turn about 3150 at 70mph.

[M&M CUSTOM]

The numbers C 25 1 you are asking about are the casting date code and nothing else.

Your axle was manufactured on March 25 of 1971.

You need the identifier code stamped into the passenger side front of the axle tube itsself to find out what was originally installed in there.

Even though the truck is as old as it is, I am willing to bet that the original gears ARE in there.

[choppy71]

I am going to list the numbers that are on the passenger side.

On the front side of the rear end -
NF 3896838

On the drivers side GM59

On the rear next to the cover passenger side there are two numbers.

7890 and 12345

Let me know if these numbers mean anything.

Thanks