Axle ratio help

[Keith Seymore]

"BJC" is the broadcast code for the axle. It represents the "pick code" for the specific axle part number, by model/series/brake system/ratio/locking vs open.

But - to decode that - you would have to have access to the secret decoder ring.

Which I do not. Edit: but the site does. See post 22 and 23 below.

K

 

[HotRodPC]

Hello everybody. I'm trying to figure out what the gear ratio is on my 84 base model 3/4 ton Sierra is. The SPID says GQ1 Rear Axle - Standard. I tried finding it online and the GQ1 isn't listed. Can someone help me out with this please? Engine is a 350, and the trans is a TH400. I'm hoping they are something for daily driving, and highway use once in awhile.
Thanks

So by 84 the small trucks got the 14 bolt Semi Float 9.5. Could be 4.10, 3.73 or 3.42. If it was a big block truck, it would have the 14b Full float 10.5 inch and could be 4.10, 3,73 or 3.21. In the Semi Floats i couldn't say which is most common cuz I know of 4 of them, and 2 are 4.10 and 2 are 3.42. Either find out what the standard ratio was for that year, or jack up one wheel, put the trans in N and count the wheel revolutions and x2 if it's a peg legger 1 wheel wonder. Just over 2 full driveshaft revolutions to 1 full wheel spin, it'll be 4.10's. If it's just under 2 full spins, it'll be 3.73 and if almost 1 3/4 spins it'll be 3.42s.

EDIT. Now that I type all that, I see it's already been discussed. But yeah !!!

 

[DoubleDingo]

This is the number stamped on top of the right axle tube. I've tried a Google search and it comes back as a table planer????

It'll be stamped on the ring gear, not the "axel" tube

 

[HotRodPC]

It'll be stamped on the ring gear, not the "axel" tube

There will be code stamped on the rear of the axle tube also. We have the decoder as a Sticky here somewhere. Retro did that for us. It might be in the GM Squarebody Library.

 

[DoubleDingo]

There will be code stamped on the rear of the axle tube also. We have the decoder as a Sticky here somewhere. Retro did that for us. It might be in the GM Squarebody Library.

True. I trust innards first though

 

[Terlingueno]

There will be code stamped on the rear of the axle tube also. We have the decoder as a Sticky here somewhere. Retro did that for us. It might be in the GM Squarebody Library.

can you find and link said decoder once you find it please?

Edit to add: Is this it?

https://www.gmsquarebody.com/threads/master-rpo-code-list-188-pages-pdf-generic.4181/

 

[HotRodPC]

Here is the link to Rear Axle Codes. Hard to see it, but it's here >>> https://www.gmsquarebody.com/threads/squarebody-rear-axle-codes.4185/<<<

 

[bucket]

Here is the link to Rear Axle Codes. Hard to see it, but it's here >>> https://www.gmsquarebody.com/threads/squarebody-rear-axle-codes.4185/<<<

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[HotRodPC]

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Well that sux. So we know it's not a G80 Gov Lok, but it could still be 2 different ratios, at least it eliminates .3.42 option. He should know if it's 4.10's or not. It'll be screaming on the freeway if it is. But as said, of the 4 Semi Floats, I've had my hands on, 2 were 3.42 and 2 were 4.10.

 

[Keith Seymore]

can you find and link said decoder once you find it please?

Edit to add: Is this it?

https://www.gmsquarebody.com/threads/master-rpo-code-list-188-pages-pdf-generic.4181/

Nope.

Option codes are different than broadcast (pick) codes.

See if this helps (post #2): https://www.gmsquarebody.com/threads/why-do-we-have-rpos-anyway.45494/

Broadcast codes are a shortcut so the operator does not have to remember the whole 8 digit part number when he is selecting the part to put on.

 

[DoubleDingo]

Well done, Lego.

Just to flesh it out a bit more: I see C20903 W LT9 with the three speed auto (non overdrive = MX1) so I'd call it 4.10:1.

K

I'd bet it's a 4.10 as well.

 

[84GMCSierra]

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So if I'm reading this right, I have 4.10 gears

 

[HotRodPC]

So if I'm reading this right, I have 4.10 gears

Not necessarily, I'd call it educated guesses or assumptions, but it's looking that way. Nothing confirmed for positive though. Best thing to do is jack one rear wheel. Spin 1 full tire revolution and count how many times the driveshaft spins. If the drive shaft spins 2 full times, safe to say you have 4.10's for sure. If it's less than 2 full turns, then it's 3.73s.

 

[HotRodPC]

Is your speedo accurate and do you have a tach in the truck? If your speedo is not accurate, then download the Speedometer app, DIGIHUD. DIGIHUD works well and I find it to be very accurate. So now if you have a Tach, lets see what your RPM is at 60mph and let us know how tall your tire is. Then we can do that math and tell you what the gear ratio is. Keeping mind, you don't have a lock up converter, so you'll going to lose 200-300rpm, but we can compensate for that and tell you for sure what your gear ratio is with that info. RPM's at 60mph and how tall your tire is, or what size tire you nave and we figure out the tire height too.

 

[Turbo4whl]

So if I'm reading this right, I have 4.10 gears

Yes

but it could still be 2 different ratios

No, he stated 1984. Year 1985 is the 3.73:1