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Discussion Starter · #1 ·
Does anyone know how you find out what gear ratio you have in a chevy c20 2wd truck....is it labeled somewhere on the vehicle or is it the VIN #?? I have a 3 spd auto (presumably th400 I am told) in a 1982 3/4 ton.
 

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fj45lvr;1618542; said:
Does anyone know how you find out what gear ratio you have in a chevy c20 2wd truck....is it labeled somewhere on the vehicle or is it the VIN #?? I have a 3 spd auto (presumably th400 I am told) in a 1982 3/4 ton.
If would be in the RPO code-sheet if your truck has one. Some do, some don't. It's usually pasted on the door of the glove-compartment. A 1/2 ton truck will most likely be 3.08 or 3.42. A 3/4 ton 3.73 or 4.10.

GU4 : AXLE REAR, 3.08 RATIO
GU6 : AXLE REAR, 3.42 RATIO
GT4 : AXLE REAR, 3.73 RATIO

GT5 : AXLE REAR, 4.10 RATIO




You can figure your axle ratio quite easily. Just jack one wheel off the ground, put the trans in neutral - and turn the wheel one full turn - and see how many turns the driveshaft turned. This is easier if you mark both - with tape, or chalk, or anything you can see.
If the one wheel off the ground will not turn on its own (with the other on the ground), then you have a posi and must test differently. But, as long as the one wheel turns, the axle ratio is as thus:
3.08 - driveshaft turns 1 1/2 times to make the wheel turn once.
3.42 - driveshaft turns 1 3/4 times to make the wheel turn once.
3.73 - driveshaft turns 1 7/8 times to make the wheel turn once.
4.10 - driveshaft turns 2 times to make the wheel turn once.

If you have some sort of posi or locking differential, and both wheels must be up in the air and turn together - then check this way:
3.08 - driveshaft turns 3.08 times to make the wheel turn once.
3.42 - driveshaft turns 3.42 times to make the wheel turn once.
3.73 - driveshaft turns 3.73 times to make the wheel turn once.
4.10 - driveshaft turns 4.10 times to make the wheel turn once.
 

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i've seen it engraved on the main gear before...but that was on a military axle...not sure if that had anything to do with it....
 

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Discussion Starter · #4 ·
Well the glove box contained a sticker with all the optional equipment installed on the truck (the option # followed by the descriptions) some of them were as follows:

1GQ1 Rear axel-standard
1VO2 H.D. Transmission OI (I think that the cut the description off for lack of space....maybe this was Oil _____??)
1MX1 3-speed automatic tr
1C6P Heavy Duty Chasis E


I guess if "1GQ1" means something in regards to the whatever the "standard" rear axel is I'll have to find out more because that number doesn't match any that I have seen listed yet.
 

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fj45lvr;1621978; said:
Well the glove box contained a sticker with all the optional equipment installed on the truck (the option # followed by the descriptions) some of them were as follows:

1GQ1 Rear axel-standard
1VO2 H.D. Transmission OI (I think that the cut the description off for lack of space....maybe this was Oil _____??)
1MX1 3-speed automatic tr
1C6P Heavy Duty Chasis E


I guess if "1GQ1" means something in regards to the whatever the "standard" rear axel is I'll have to find out more because that number doesn't match any that I have seen listed yet.
MX1 is the TH350 transmission (never used OEM with the 6.2 diesel).
GQ1 does stand for "standard axle." I find that often on GMs.
C6P means you have a 1/4" thick frame.
VO2 is the heavy duty radiator with built in trans. cooler.
 

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Discussion Starter · #7 ·
jdemaris;1622621; said:
MX1 is the TH350 transmission (never used OEM with the 6.2 diesel).
So you think that the buyer had to make a custom request for this trans over the 400 then....weird??

I found a site ( http://www.c2e.info/rpocodes/gmrpocodesGthroughM.html )
showed that MX1 was :
MX1 : MERCHANDISED, TRANS, AUTO PROVISIONS

most of the items on my sticker are just generic and not specific information such as "standard ratio".
 

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fj45lvr;1623081; said:
So you think that the buyer had to make a custom request for this trans over the 400 then....weird??

I found a site ( http://www.c2e.info/rpocodes/gmrpocodesGthroughM.html )
showed that MX1 was :
MX1 : MERCHANDISED, TRANS, AUTO PROVISIONS

most of the items on my sticker are just generic and not specific information such as "standard ratio".
I know of many 6.2 diesels that got transmission swaps at the dealer - but I never heard of a dealer chaning the RPO code-sheet. I DO have a 91 diesel Suburban with some RPOs that were added by a dealer- and they are taped on.

I have many different databases with GM RPO codes, and they don't all agree. I have several that list as follows:

MX0 and MX1 - Merchandising option only - do not catalog 1982 - 1988

And, all of the following are RPO codes for TH350 trans.

MX2 (1976 - 1984) , MX3 (1976 - 1981), MX5 (1982 - 1983)
M33 - 1976 - 1981, M38 - 1976-1981
 

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I new a guy that bought an 82 K-20 brand new in California with a 6.2 and it had a turbo 350 tranny in it. Could have been first run of production as that is the only one I have ever seen from the factory that way.
 

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D.Camilleri;1625750; said:
I new a guy that bought an 82 K-20 brand new in California with a 6.2 and it had a turbo 350 tranny in it. Could have been first run of production as that is the only one I have ever seen from the factory that way.
I suspect that GM made several "odd-ball" special order vehicles. In 82, I saw - and drove - a brand-new Chevy full-size station wagon with a 6.2 diesel - yet according to GM standard records, it was never made.
Same goes for my 83 K5 diesel Blazer with a four-speed manual overdrive trans -that I bought from the original owner.
 

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fj45lvr;1618542; said:
Does anyone know how you find out what gear ratio you have in a chevy c20 2wd truck....is it labeled somewhere on the vehicle or is it the VIN #?? I have a 3 spd auto (presumably th400 I am told) in a 1982 3/4 ton.
I've found most of the 3/4-tons 6.2L diesels to have 4.10 ratio.
 
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