Determining Rear Gears

[cherokeejohn]

Is there a way to determine what rear gears I have from the VIN or the SPID? I've tried climbing under the truck read (or even find the numbers on the axle tube), but there's 42 years of farm corrosion down there and I couldn't find a darn thing. Anyone know if it can be done? John

[70SBC10BOB]

Jack up the truck and put jack stands under the rear end (with tires off the ground).
Put the trans in neutral.
Put a mark on the tire that you can see while you lay under the truck.
Turn and count the revolutions of the drive shaft until the tire makes a complete revolution.

Example: turn the drive shaft 3 and a half times = 3.50 gears
turn the drive shaft 4 and a little bit = 4.11 gears

Hope this helps.

[cherokeejohn]

Thank you 70SBC10BOB. I'll give that a try tommorrow. Fortunately the trucks already on jack stands undergoing a trans swap, so this shouldn't be a problem.

[hamjet]

If it came from the factory with 350 CI and 3 speed auto, standard gear ratio would be 307s.. unless the spid says something different or someone changed it out in the last 40 years.. I never had any luck turning the tires and counting with open gears, one wheel goes one way, the other goes other way.. pull the rear end cover and you will see numbers stamped on edge of ring gear. divide them up for the ratio...

[texasld]

I just so happened to be wondering this question. Thanks for asking it!

[THE BOSS]

they way I do it is jack up 1 rear tire and turn it twice then count the drive shaft revolutions.

[Keith Seymore]

Originally Posted by cherokeejohn

Is there a way to determine what rear gears I have from the VIN or the SPID? I've tried climbing under the truck read (or even find the numbers on the axle tube), but there's 42 years of farm corrosion down there and I couldn't find a darn thing. Anyone know if it can be done? John

Can't tell from the VIN (gear ratio is not coded in the VIN).

It will show up on the SPID if it is an optional ratio, with the optional RPO ratio and name.

Often if it just has the base ratio then it will not show up, or will show RPO GQ1 (standard axle) but will not specify what the ratio is.

K

[slikside]

Originally Posted by hamjet

If it came from the factory with 350 CI and 3 speed auto, standard gear ratio would be 307s.. unless the spid says something different or someone changed it out in the last 40 years.. I never had any luck turning the tires and counting with open gears, one wheel goes one way, the other goes other way.. pull the rear end cover and you will see numbers stamped on edge of ring gear. divide them up for the ratio...

Originally Posted by THE BOSS

they way I do it is jack up 1 rear tire and turn it twice then count the drive shaft revolutions.

This is how I did it also. With an open diff, you have to keep 1 tire on the ground, and spin the other 2 revolutions. Count driveshaft revs in those two turns of the 1 tire.

And just like ham says, mine came equipped as stated, and I got just a shade over 3 revs of the drive shaft....3.07 or 3.08, dunno which.

Now, if you have a posi, obviously both wheels must be off the ground, and I think you only have to do 1 rev of the tire, and count...

[LockDoc]

-
Here is some info I posted a while back. If you scroll down to post #8 it shows where the code number is stamped on some rear end housings.

http://67-72chevytrucks.com/vboard/s...d.php?t=578793

LockDoc

[Keith Seymore]

Originally Posted by LockDoc

-
Here is some info I posted a while back. If you scroll down to post #8 it shows where the code number is stamped on some rear end housings.

http://67-72chevytrucks.com/vboard/s...d.php?t=578793

LockDoc

Thanks, Doc. If you ever come across that information for '73-87 I would be interested.

K

[LockDoc]

Originally Posted by Keith Seymore

Thanks, Doc. If you ever come across that information for '73-87 I would be interested.

K

Will keep you in mind Keith.

LockDoc

[SS Tim]

Originally Posted by Keith Seymore

Thanks, Doc. If you ever come across that information for '73-87 I would be interested.

K

Keith,
All you need to do is go over to the GM parts wiki search or open a GM parts catalog. Engine codes and front or rear axle code identification tables can be found by year in the information section.

At the wiki search, http://www.gmpartswiki.com/query

As an example...

axle code

parts catalog

leave blank "single book"

chevrolet truck

1973

Results included this page TP23 from Parts Catalog P&A 51

Just refine from there and you will have no trouble. Of course not all manuals are there and some codes seem to be missing, at least for the 67-72 I am asked to research there have been a few.

[special-K]

Is it on the road? Take it down 140 at 65 mph. If it wound out it's 4.10s,if it's got some decent revs and still some pull it's a 3.73,and if it's humming right along with ease it's a 3.08
I think 3.73 came with 3spdsl.

Pull the rear cover,count the teeth or look for the numbers on the ring gear side,reinstall cover,and put fresh fluid in. Most likely needs a change anyway.

[Keith Seymore]

Originally Posted by SS Tim

Keith,
All you need to do is go over to the GM parts wiki search or open a GM parts catalog. Engine codes and front or rear axle code identification tables can be found by year in the information section.

Just refine from there and you will have no trouble. Of course not all manuals are there and some codes seem to be missing, at least for the 67-72 I am asked to research there have been a few.

Perfect! Thank you -

K