[70 Dream]

Will 3.08 rear gears work OK with my 1970 400 with AC and TH400?

It came standard with 3.23s because of factory AC, but I am assuming they are gone, because the axle code is XH, which equates to 3.55s, so I assume 3.55s are in there now. I'm running about 3,000rpm @ 60mph.

Anyway, my neighbor gave me a set of 3.08s - a ring and a shaft with teeth on them - the bigger ring has 40 teeth, and the smaller ring on the shaft has 13 teeth, so 40/13=3.08?

My two questions are:

Why would someone swap out the entire axle on my car?

Are 3.08s too tall? If no, why didn't Pontiac offer it, then?

Thanks, guys,

Harry

[LouisianaGuy]

The rear could have went out at one time and some one decided to swap the whole rear because it was easier and they did not have to pay some one to set up new gears for them. I would pop the cover and see what gear is actually in there. The 3.08's will work and be great on the highway, but will be lacking a lil bit outa the hole. I swapped from 3.73's to 3.08 in my 455 TA and lost a few tenths in the quarter...not alot of difference for a 455 :P So I comprimized and put in 3.23's
Mike

[70 Dream]

Thanks, LAGuy,

How do I tell what's in there, what am I supposed to look for? Do I make the same calc as I did with my friend's gears?

Also, how can you tell if you have posi?

[INJUNTOM]

if you are just cruising, a 3.08 will be nice.

i've ran 2.73 with a stock 400, and ran very well.

to check for a posi, you can jack the rear up, and turn one wheel...if the other turns the same way, it will be a posi. if it turns the other direction, then it is an open diff.

[thumper78]

to tell what ratio you have without opening the axle:

• Jack up the rear of car, support on stands
• Mark the driveshaft and the inside of one of the wheels
• Rotate the wheel.
• Watch the driveshaft mark and keep count
• When wheel has made 1 revolution, note number of turns by driveshaft. Be sure to note partial turns

If driveshaft turns just over 3 turns for 1 wheel turn, ratio would be 3.08.
3 1/2 turns would be 3.55. Just over 4 turns would be 4.10. 2 3/4 turns would be 2.73

You get the idea?

[Jack Gifford]

Thumper told you the easiest way to check the ratio. However, with that method and a non-posi differential, you need to hold one wheel while you turn the other one- or simply just jack up one. But the differential will cause the pinion and driveshaft to turn only half as far as it would if you were rolling down the road with both wheels turning. So if it's a 3.08 and you turn one wheel one turn, the pinion will only turn 1.54 turns.

[Thumpin455_GTO]

What rear end do you have under the car? 70 GTO's had an 8.2" rear with the 400, if that ring and pinion arent for the rear end you have, its useless to you.

As far as 400 with 3.08s they work fine in a street car, shouldnt be a problem for you. We have run 3.08 behind 400s and 455s makes for nice cruise rpm, not too much loss in ET from 3.55

[70 Dream]

Thumper, I don't know for a fact what the rear end is.

As mentioned, the axle code is XH, which equates to a 3.55, so I'm assuming I have 3.55 rear gears.

The ring and pinion my neighbor gave me came out of his 74 Firebird.

[Pontiac Poor]

Why not jsut pop the rear diff cover and look? it only takes about 10 minutes. Then you will KNOW what gear you have and if it is a posi. No guessing involved. When was the last time the gear lube was changed? If you don't know, it might be a good time to change it.

[thumper78]

Originally Posted by 70 Dream

Thumper, I don't know for a fact what the rear end is.

As mentioned, the axle code is XH, which equates to a 3.55, so I'm assuming I have 3.55 rear gears.

The ring and pinion my neighbor gave me came out of his 74 Firebird.

OK 70 Dream. Must of misread your post.

Just thought I was answering your question in post #3. Figured you were looking for a way to easily verify what ratio is actually in the rear.

[Thumpin455_GTO]

74 Firebird would have an 8.5 rear rather than the 8.2, so the gears probably wont fit. I remember something about some 72 A bodys getting the 8.5 rear, but I have yet to see one. I never had much luck finding 8.2 posi carriers in that would take 3.08 gears, you can get new ones for 3.90 up. I havent messed with 8.2s in forever though, so they may be a chance you can get the lower carrier and gears now. I went to 12 bolts and 9" rears in my A bodys.

[Pontiacer]

Cutlass and Skylark got the 8.5 in '71. Pontiac didn't get 8.5 until '73. With cover off, the ring/pinion count is stamped on edge of ring gear. '74 Bird will have 8.5 rear.

[70 Dream]

Well, now I'm stumped,

The car was in the air at the muffler shop over the weekend, so I turned one of the rear wheels, and the other wheel went the opposite way . . . doesn't that indicate an open rear end?

So, that means I have an open rear end? The axle code as mentioned is XH, which is a posi code, so why don't I have posi?

So, if I take off the differential cover, what am I supposed to be looking for?

And if I want posi, where can I get a 3.08 posi? As Thumper said, are these hard to find?

Thanks,

Harry

[form406]

all the XH codes i see are not posi they are open...you should open and check anyway... http://www.teufert.net/pontiac/rears.htm

[70 Dream]

Form, thanks for the chart . . .

The chart shows code XH as being in the posi column, which makes sense because the 1970 order form shows all of the Safe-T-Track (posi) axle codes as starting with X, and non-Safe-T-Track codes starting with W.

I am wondering how all of the ones you've seen are open?

I do know that my car had factory AC, so it would have come with 3.23 gears, that's how I know my axle is not original. It is very interesting that someone would have put in an XH axle, and then swap out the posi 3.55 differential later for an open.

The differential looks like it's easy to get to, what am I supposed to be looking for?

[napster]

70 Dream,

I will be very interested in what you find. I run 3,000 RPM at 60 - 65. My 1970 GTO ( 4Spd) came with A/C and a 3.23 open rear from the factory. I know that the car has posi right now. It's making me wonder if I have 3.55's.

Also, I have a 1978 S/E T/A ( Auto) with A/C and 3.23 posi rear. This car also runs 3,000 grand @ 60-65.

Nap

[70 Dream]

Yes, I will report back as soon as someone tells me how to take off the differential cover, and what I'm supposed to be looking for . . . .

(Never done it before, kind of scared, but willing to try)

[form406]

actually i guess i see XH i see both columns...but your year in the posi column...best thing to do is pull the cover...all you do is slowly unbolt it and let fluid drain into container its easy...then refill it...

[mj64car]

Your speed at a given RPM also depends on what diameter tires you have. My stock 3.08 is equivelent to about 3.28 with the smaller tires on the car now. Check your size tires to what were stock.

[form406]

mj64 what size tires do you have on the rear? with my 3:08 i have 265/50/15's which seem to figure out to?