TH400 & 2:73

[RAD77C10]

I was checking codes on my 77 C10 short bed, found that it has a TH400 behind a 350ci. I found no code for the gear ratio, so I jack it up & turned the driveshaft & counted the tire rotation. Looks like it has a 2:73 gear ratio which is high for a truck that said it has a heavy duty package, does that sound right? I guess I need to pull diff. cover & see what is stamped on the ring gear. That just doesn't make sense for a heavy 1/2 ton that's bone stock!

[LTZ C20]

Is it possible you miss counted or mean 3.73? I wouldn't be surprised at all if with the options you described it has a 3.73 ratio.

[Stewart G Griffin]

It doesn't make sense, yet combinations like that do exist.  i have the "heavy duty" rear springs AND a 2.56 rear gear----it can support the weight, it just can't move it.

[RAD77C10]

I pulled the diff. cover today, found it to be 3.42! I must of counted wrong or something! I'm good with 3.42 in 10 bolts.

[Stewart G Griffin]

The truck i got the 4.3 out of came backed with a TH-400! 

[Blazin]

3:42s in a 77? 3:08s, 3:73s were the common gear for that vintage. Didn't start seeing 3:42s being common until 81 /82.

[fitz]

Wouldn't a 77 C10 come with a 12 bolt?

[RAD77C10]

Your right, it is a 12 bolt.

[roundhouse]

3:42s in a 77? 3:08s, 3:73s were the common gear for that vintage. Didn't start seeing 3:42s being common until 81 /82.

could have been swapped
The 10 and 12 bolts are very weak and usually don't last long


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[Stewart G Griffin]

could have been swapped
The 10 and 12 bolts are very weak and usually don't last long

Wha?

[HAULIN IT]

You can't rotate & count like that with an open wheel dif, the spider gears turning alter the accuracy. The 3.40 geared 12 bolt is a rather popular gear in the '76-'79 trucks...may be a geographic area thing, but many trucks around me have 3.40 gears. I've owned 4  1/2 ton square bodies. 3 of them came with 3.40's...the one was a parts truck that had a 4x4 front from another truck laying in the bed, it also was a 3.40. The other one came with a 10 bolt & I switched it out to a 12 bolt...yep! 3.40's 'cause they were what I found.
There are some odd-duck combo out there that were factory built. I knew of a guy that had a "Tradesman" or "Outdoorsman" truck...something like that (Trees/Mountains on a decal on the bedsides). It was about an '82(?) long bed 4x4 with a 4.3 V6, TH400 & from what I remember, the first Aluminum GM transfer case I had seen in a full-size truck. Lorne   

[roundhouse]

could have been swapped
The 10 and 12 bolts are very weak and usually don't last long

Wha?

ever go to a junkyard ?
THe first thing that sells is the back axle

In fact on a lot of square bodies.  The back axle is all that's missing


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