Gear ratio identification

[84SierraClassic]

Hey guys. I am trying to identify what my gear ratio is in my rear axle, without having to tear it apart. I have tried looking it up with the RPO code, but all I can find is GQ1=standard axle ratio. Now, looking at RPO codes for the past 2 days I have found that gear ratios have codes GT*, or GU*. Now, I only have 1 sticker with RPO codes on the vehicle, on the left inner fender. No sticker with RPO codes in the glove box. So I am at a bit of a stand-still.

cliff notes:1984 GMC SierraClassic 1500. All stock with the only exception being the motor. Only RPO code I have to go off of is GQ1. Rear diff is GM Corporate 10-bolt.

I will get tire size info, and what RPM at what speed once I get a tach installed. There is a way to find out what gear ratio is by some formula isn't there?

[RandyP]

Should also be a metal tag on the housing that I.D.'s it, I think.

[84SierraClassic]

Oh alright. I'll take a look later tonight if it's not raining. Thanks for the reply

[silvereagle07]

Does it have the original seat in it? If so, look underneath in the springs of the seat. Some trucks had the build sheet stuffed in the seat springs. I have the same year GMC and the gm corp 10 I had was a 3.08 rear gear. Here's a breakdown of the codes incase you don't know. I hope this helps.
GU2 2.73
GU4 3.08
GU6 3.42
GT4 3.73
You can get a close guess by putting truck on jackstands, in neutral, mark spot on tire, mark differential flange, rotate both wheels at same time, and count drive shaft revolutions to one wheel revolution. It will get you close without tearing anything apart.

[85Bowtie]

Jack up the rear of the truck with the truck in Neutral. Mark the drive shaft and turn one tire one complete rotation and count the number of times the driveshaft turns. If it turns 2 3/4 times, you have a 2.73. If it turns a tad over 3 times, a 3.08, not quite 3 1/2, a 3.42 etc.

[84SierraClassic]

Sweet, I will definitely be lookin' for that build sheet. As far as I know nothing was taken out of the truck, it's all still factory everything with the exception being the motor (Came with a 5.0L and the OO had a 5.7L installed after putting 40,000 on it)

[84SierraClassic]

Cool. Thanks for all the replies guys. I remember hearing from somewhere that you could find your gear ratio by turning your rear wheel and watching the drive shaft spin.

Quick clarification point: With the backend jacked up, truck in neutral, does it matter what side to turn, Left or Right? I will be doing this alone so it would be hard to turn both wheels at the same time.

[85Bowtie]

Originally Posted by 84SierraClassic

Quick clarification point: With the backend jacked up, truck in neutral, does it matter what side to turn, Left or Right? I will be doing this alone so it would be hard to turn both wheels at the same time.

No, the other will most likely just turn backwards

[Keith Seymore]

Your assessment is correct: GQ1 is the "base" axle ratio for that powertrain combination, and it doesn't tell you specifically anywhere what that ratio is.

The only way to know without doing any physical checks would be if you had the powertrain charts which showed your specific combination, or if you can find your individual build sheet. (There might be a powertrain chart in either the brochure or trailering guide - keep your eyes open for one of those).

K

[baclay9]

I tried that on mine and the driveshaft turned at the same rate as the wheel. 1:1. what's up with that? I am pretty sure my other wheel was turning the same way as well. Not opposite.

[N2TRUX]

Originally Posted by baclay9

I tried that on mine and the driveshaft turned at the same rate as the wheel. 1:1. what's up with that?....

They will both turn the same, but the driveshaft will turn more revolutions than the wheel. Put a mark on the outside of the wheel, and one on the driveshaft. Now turn the wheel one full revolution and count how many times the mark on the driveshaft turns.

[baclay9]

Mine turned 1:1. As in one driveshaft revolution per wheel revolution. I tried it on both wheels. Are you sure one tire doesn't have to be on the ground or something.

[85Bowtie]

Are you sure it's in Neutral?

[baclay9]

Originally Posted by 85Bowtie

Are you sure it's in Neutral?

Yep

[BuckB78GMC]

Put the rear axle on jackstands. Mark one of the tires on the inside and then mark one of the U-joint caps on the rear yoke. Then, turn the driveshaft by hand and count how many turns of the driveshaft equals one turn of the tire. 4 to 1 equals 4.10:1 or 4.11:1, 3 and 3/4 to 1 equals 3.73:1, 3 and 1/2 to 1 equals 3.55:1 and so on...