Axle ratio help

84GMCSierra · Jan 13, 2025

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

legopnuematic · Jan 13, 2025

You must be registered for see images attach

Or pop the rear cover and count the teeth/find the numbers on the ring gear to verify. If you haven't changed the gear oil in your ownership it's probably time to do so anyways. Gear oil is cheap compared to rebuilding a rear axle.

Keith Seymore · Jan 13, 2025

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

Keith Seymore · Jan 13, 2025

84GMCSierra said:
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

Correct. For base axle GQ1 the SPID shows GQ1 but doesn't specify the numerical ratio, because it varies (as shown on the Powertrain Chart).

For an optional ratio: The SPID will show the RPO and call it out by name (ie, GU6 = 3.42, GT4 = 3.73, GT5 = 4.10 or whatever).

K

84GMCSierra · Jan 13, 2025

Keith Seymore said:
Correct. For base axle GQ1 the SPID shows GQ1 but doesn't specify the numerical ratio, because it varies (as shown on the Powertrain Chart).

For an optional ratio: The SPID will show the RPO and call it out by name (ie, GU6 = 3.42, GT4 = 3.73, GT5 = 4.10 or whatever).

K

So I need to pull the cover and count teeth to find out what it is?

75gmck25 · Jan 13, 2025

Sine you apparently have a standard axle, without GovLock or other limited slip, you can determine ratios by counting wheel revolutions and driveshaft revolutions.

This one site with an explanation of how to do it. https://www.moderndriveline.com/how-to-determine-your-rear-axle-ratio/

mtbadbob · Jan 13, 2025

Put it on a lift, turn your rear wheel one revolution, and count how many times the drive shaft rotates in that same revolution.

Ricko1966 · Jan 13, 2025

mtbadbob said:
Put it on a lift, turn your rear wheel one revolution, and count how many times the drive shaft rotates in that same revolution.

This won't work if it's open diff,you can push the car on the ground,1 revolution stop every 1/4 turn check and mark your drive shaft. Or jack up 1 wheel and turn it through 2 revolutions and count your driveshaft turns. Nevermind it's 4.10,look at the power train chart @legopnuematic put up for you. All 350s with the mx1 transmission and gq1 axle are 4.10

mtbadbob · Jan 14, 2025

Ricko1966 said:
This won't work, if it's open diff,you can push the car on the ground,1 revolution stop every 1/4 turn check and mark your drive shaft. Or jack up 1 wheel and turn it through 2 revolutions and count your driveshaft turns. Nevermind it's 4.10,look at the power train chart @legopnuematic put up for you. All 350s with the mx1 transmission and gq1 axle are 4.10

Why won't it? It's worked for me before on my Olds...

Ricko1966 · Jan 15, 2025

mtbadbob said:
Why won't it? It's worked for me before on my Olds...

It will work on a posi but not an open differential because of how the diff it's self works on a posi both wheel spin same speed on a open diff they don't.

59840Surfer · Tuesday at 2:40 AM

Ricko1966 said:
It will work on a posi but not an open differential because of how the diff it's self works on a posi both wheel spin same speed on a open diff they don't.

Partly true ----- on an open differential, the side and spider gears change the revolutions in relation to the driveshaft - so the numbers won't be anywhere near accurate.

You can get in trouble with the Gov-Loc® system too because you CAN turn one axle slowly and the posi-effect won't hook up ... and again you're counting side gears and spiders.

The tooth count is always on the ring gear --- simply like this as an example ---> 38:12 or 33:07 --- something like that --- in form, but not necessarily those numbers.

Happy hunting!

cornholeeo · 2025-01-22T18:17:02-0500

Ricko1966 said:
It will work on a posi but not an open differential because of how the diff it's self works on a posi both wheel spin same speed on a open diff they don't.

so youre trying to tell me that if you have rr wheels off the ground and the trans in neutral, spinning the drive shaft by hand that you cant get a close to accurate ratio. There is no slip from drive shaft output to the wheels.Any slippage is from side to side

Ricko1966 · 2025-01-22T19:36:04-0500

The spider gears and axle end gears come into play. You could rotate the driveshaft and watch the tires if they both spin the same direction equally you can count number of drive shaft turns to 1 turn of the wheel. If 1 wheel spins clockwise and the other spins counter clockwise,you will have to hold 1 tire stationary and turn the driveshaft,count how many turns to make 2 turns of the tire.If you want to just turn the driveshaft and count turns without one wheel stationary or both wheels spinning the same direction,you are more than welcome to try.

Axle ratio help

84GMCSierra

Full Access Member

Attachments

legopnuematic

Licensed Junk Dealer

Keith Seymore

Full Access Member

Keith Seymore

Full Access Member

84GMCSierra

Full Access Member

75gmck25

Full Access Member

mtbadbob

Full Access Member

Ricko1966

Full Access Member

mtbadbob

Full Access Member

Ricko1966

Full Access Member

59840Surfer

Full Access Member

cornholeeo

Junior Member

Ricko1966

Full Access Member

Similar threads

Latest posts

Forum statistics