[Axle07]

Didn't know where to put this so I figure this would be my best bet. I'm trying to figure out what rear gears I have on my 77 T/A. I looked for the RPO code under the hood but its not there so I'm guess it's on the build sheet which I don't have So is there any way to figure out what I have without getting under there and pulling the back of my rear end off? lol Or possibly get a hold of a build sheet somehow? As far as the car its a 77 T/A, 400 cid, 3 speed automatic. Any help is appreciated!

[newtoponcho]

Hi there,
I'm not 100% sure of the location of your "spid" label (service parts identification) on your car but they are usually located in the glove box,trunk lid or possibly the center console.Finding the "G" rpo codes would be the easiest option.next option would be stampings on axle housing I believe on the right side.last option, pull the cover & count.
Hope that helps.Good luck.

[Axle07]

thanks

[77 TRASHCAN]

I've never seen the ratio advertised anywhere on your car. I didn't figure mine out until I got the diff. cover off, the first time.
If you have the lowest compression/horse power version of the 400 that year, the harmonic dampener does not have the outer ring (usually) . You probably have the 2.41:1 gear that my car has in it. In my opinion Pontiac probably wasn't too proud to advertise that gear ratio, didn't really want the customer to know...
My car ran pretty well when it was newer... got great MPG, still does...

[newtoponcho]

I forgot to mention, the information you find will only be correct if the gears have not been changed.You may be pulling the cover after all.There is a way to get a rough idea,by jacking up the rear of the car and marking the pinion yoke to diff, and backing plate to tire. then count the amount of turns the pinion yoke makes to one complete turn of the tire.Hope that helps.

[Axle07]

I figure 2.41 is about right lol judging from the pull of the car. It wont really break the tires loose from a dead stop, of course in reverse it will do it in a heartbeat. idk if they are 2.41 I'm looking for something better. thanks for the help

[TransAm525]

The code is on the front side of the passenger side axle tube, a few inches from the differential housing. A wire brush may be required to make the code visible. You will have one of the following:

PX for 3:23
PW for 3.08
PT for 2.56
PS for 2.41

If you pull the differential cover, the pinion gear will have two numbers stamped into it. This is the tooth count of the ring and gear set. If you divide the higher number by the lower number, you will have your exact ratio.

If your car has the W72 engine package, you have 3:23 gears unless they were changed at some point.

[OINK 1]

It Should be a posi unit. To tell, jack up both rear wheels, get a friend to help.Place in neutral. If you turn One wheel the other should turn in the same direction, if it does it's a posi !! If it goes the other way , it's not a posi. If it is a posi, now turn the rear wheels 1 full turn and count how many times the drive shaft goes around. 1 full turn on wheels, 2 1/2 on the drive shaft( close enough) it's a 2:41. 1 on wheels 3 1/4 on shaft= 3:23, 3 1/2 turns 3:55's, almost 3 1/2 turns = 3:42 . 3 3/4 turns= 3:73's etc. etc. If it's not a posi, remove cover count gear teeth on ring gear and pinion gear, divide ring # by pinion= ratio !!

Pontiac Gregg

[b-man]

Originally Posted by OINK 1

It Should be a posi unit. To tell, jack up both rear wheels, get a friend to help.Place in neutral. If you turn One wheel the other should turn in the same direction, if it does it's a posi !! If it goes the other way , it's not a posi. If it is a posi, now turn the rear wheels 1 full turn and count how many times the drive shaft goes around. 1 full turn on wheels, 2 1/2 on the drive shaft( close enough) it's a 2:41. 1 on wheels 3 1/4 on shaft= 3:23, 3 1/2 turns 3:55's, almost 3 1/2 turns = 3:42 . 3 3/4 turns= 3:73's etc. etc. If it's not a posi, remove cover count gear teeth on ring gear and pinion gear, divide ring # by pinion= ratio !!

Pontiac Gregg

If it's not a posi (open differential) just leave one tire on the ground to hold it from turning.

Then go through the same driveshaft turning and counting as Gregg has outlined.

In this instance (open diff) use 2 full turns of the wheel that's not on the ground instead of only one turn with a posi, due to the 2:1 ratio of the differential side & spider gears.

Myself, I always turn the shaft instead of the wheel, same outcome.

There's really no need to remove the cover to check your rear gear ratio. If you have a rear end with a drop-out center section (the popular Ford 9" would be one) then there is no easy-to-remove cover like on a GM 10 or 12-bolt rear, counting the turns of the driveshaft is always the easiest way.

[Overkillphil]

If indeed a non-posi (doubtful), I use an even easier method. Reach underneath and put a mark on some part of the driveshaft or U-joints. Then, on level ground place two opposing wheel chocks so that a tire can only rotate on revolution before hitting the opposite chock (Tip: eyball a valve stem clock position to set up the one revolution distance). Have someone slowly push the car in between the chocks while you count the number of turns on the shaft.

My bet is on a 2.41 though for your stated 77 combo.

[Axle07]

last time i was under there the only tag i saw coming off the rear said limited slip differential. Gonna check tomorrow to see what i got though.

[OINK 1]

The one wheel on the ground idea will not work, the little spider gears inside the single track unit will also turn, multiplying the rear ratio, giving incorrect results.

Pontiac Gregg

[b-man]

Originally Posted by OINK 1

The one wheel on the ground idea will not work, the little spider gears inside the single track unit will also turn, multiplying the rear ratio, giving incorrect results.

Pontiac Gregg

There's no need to apologize for me Gregg, because you're the one who's mistaken here.

Example for 3.50 gears (a common Ford 9" ratio):

Leaving one tire on the ground (open non-posi rear) and turning the driveshaft 3-1/2 turns will always result in the tire that's off the ground making exactly 2 full revolutions. As already stated, this is because of the 2:1 ratio of the differential gears.

Of course the little spider gears are turning inside when using this method, but they don't skew the results one bit.

Maybe you should try this before coming on here to tell me I'm wrong.

As I already have stated, there is absolutely no need to remove the cover (or the entire gear carrier on a drop-out rear) to check the gear ratio.

b-man Bart