72442TwiceOwned

I’ve got a 72 442 with the original U code 455 and original TH400 transmission in it and I’ve owned it twice. Someone between my ownership stints removed the original 2.73 rear end. It was replaced with a 12 bolt and I was told it was a 3.23.

First, the car was on a lift yesterday for an oil change. I couldn’t venture in and get a detailed look but can anyone tell from these pictures if it is an Olds or (I’m guessing more likely) a Chevy or other GM rear axle?

Second, with the engine being the correct 72 455 I can tell you that the RPMs at 70 mph is around 3600. Does that sound right for a 3.23 differential ratio or is this more like a 3.73 ratio or somewhere around that? It seems to rev a good bit higher than I remember the original 2.73 doing.

-On a side note, the car was on the lift for an oil change because there was a little gas in the oil due to backfiring. The MSD ignition control module started going out recently. I got off the interstate one night and hit the ramp at about 70. I let off the gas and the car backfired hard about 8 times taking out the mufflers (gaping hole in the back of the left one and a split in the middle of the right one). No major loss - the system (what’s left of it) is about 30 or more years old. Fortunately, I have a new Gardner system on hand just waiting to go in once I’m sure I’m past any backfiring.



Just an aside…Damage from the backfiring when the MSD ignition control module started going out.



And one shot off the underside of the car, just because I had it.

Thanks for any feedback in advance!

monzaz

That is a Oldsmobile 8.5 12 bolt cover 10 bolt ring rear end 1967 --1970 era
On the MSD please check your power connection to the MSD box and or distributor.
IF the power goes intermittent the car will load up and then when it comes back will ignite all that fuel in the exhaust like it did.

CHECK your power connections...If they are good the MSD box could be taking a crap.
I had a loose battery cable that cause this same thin on the 6AL MSD box and it backfired too bad. did not get a blow out but scared the crap out of me...lol.
Jim

Vintage Chief

Suggest you perform a thorough and proper tune-up ASAP. Backfire through the exhaust on deceleration is most often the result of either incorrect A/F mixture ratio or incorrect timing. Often running too lean which may appear contrary to popular belief when residual fuel is burnt in the exhaust system under a rich condition at idle. At any rate, not to argue the merits/notions of which it is (rich or lean), a healthy accurate tune-up according to specs is appropriate.

72442TwiceOwned

Thanks very much, Jim! I’m glad to know it’s at least an Olds differential. The shop confirmed power was getting to the MSD unit and then installed a new ignition control module and coil due to no spark so I think I’m beyond the backfiring now (fingers crossed). I’ll have to get them to check and make sure the power is consistent, though. It still wants to fall in its face every now and then when I gun it. Those backfires sure made me jump, too!

Any thoughts about the RPM question? If not, no worries. Thanks for the help!

John

72442TwiceOwned

Thanks for the tips. It was tuned up in the last 6 months with new plugs. They said the timing was off 6 degrees but the previous tuneup (different shop that originally installed the MSD) only had about 2000 miles on it and ran great when the car was parked in 2005. I’m wondering if where the timing was before they adjusted it is where it needed to be. I don’t know where it was or where it is now but plan to find out. I think they may have backed it up too much but I’m no mechanic - the car just feels a little more sluggish now. They deemed the plug wires to be in “decent” shape - whatever that means. They are Taylor wires.

woodie582

Tire diameter needed to factor into the RPM equation.

72442TwiceOwned

Thanks. 245 60r14 measures right at 24.5 inches.

woodie582

3.73 Seems about right..

72442TwiceOwned

When you mentioned the formula I realized I could have checked online for that. This is what I found so 3.73 seems to be right. That’s right at exactly where my factory tach is.

Thanks again!

1969w3155

Undersized by about 1.5"-1.7" for your car, but that would mean a few more revs on the highway. I am pretty sure that you have 3:42's in the back of it. I had 3:42's in my '71 Supreme with 245-60x15 at the rear, which is close to the original dia., and I ran about 3200-3300 @ 70mph.

Fun71

My car with that size rear tire and 3.23:1 gear ratio turned around 3400 RPM at 70-ish MPH.

Fun71

Except there never was a 3.73 ratio for the O-Type rearend. 3.42 and 3.90 are the closest.

joe_padavano

Measure again. The 245/60-14s are 25.57" diameter. Still waaay too much daylight in the A-body wheel wells. Consider the 225/70-14s. An inch taller (and just about the same OD as the OEM F70x14 tires) and barely 3/4" narrower than the 245s.

Fun71

Joe, that is about what I measured by placing a rule against the tire. There is a difference between what the manufacturer states and what the tire measures when sitting with the weight of the car on it. I recall reading somewhere the manufacturer’s measurement is a tire spinning at speed.

joe_padavano

Actually the numbers I posted are the nominal diameter. Yes, there is a number called the "rolling radius" which accounts for sidewall flexing under the weight of the car. Radials flex more than bias ply tires, resulting in a smaller rolling radius for the same nominal overall diameter.

72442TwiceOwned

Many thanks to all of you for weighing in on this! I went with this tire size years ago because the car has 1/2” drop springs in the front. Anything much taller seemed to be all up in the fender well. But, I can’t deny they are on the smaller size height-wise. I’m going to study this more closely when I get closer to needing tires. I may also go back with the stock spring height if I get new new shocks or do any other front-end work.

Also, my tic-toc-tac is original and has never been out of the dash. Are they pretty accurate as a general rule? All the discussion has made me curious.

I plan to take the car back out and just reconfirm RPMs at 60 and 70 and then recheck tire height while it’s hot. Thanks again for all the help with this!

BangScreech4-4-2

No.

72442TwiceOwned

Thanks. Thinking about it, I probably should have said accurate but that may not change the answer.

Dream67Olds442

Also, with tires that are not OEM size, there's a good chance your speedometer reading is not accurate. You could check that with a portable GPS is you have one, or know someone that does.

vCode442

Waze app is helpful to check speed/ speedometer

72442TwiceOwned

Great point. Thanks for that tip. I don’t have one but I’ll do some checking into it.

72442TwiceOwned

Thanks. I’ll have to take a look at that app. I’m curious now to see how off it is.

72442TwiceOwned

So, I’m picking this thread back up since I read up and found that the axle housing should have a code on it that reveals the ratio.

Based on RPMs at 70 (around 3600) I had suspected around a 3.42 and I think others that responded with 3.23, 3.42 or possibly 3.91. However, the code on mine is R2 which I understand from an old book I have to be a 2.56 ratio (non-posi) across 69-71 year models.

My questions are
1) Are the housings on all O rear differentials all the same so that any of the ratios (3.23, 3.42 and 3.91) could fit into what was originally a 2.56?
OR
2) Is it more likely someone had to replace an axle housing, used whatever they could find and my differential has whatever gearing actually came in it originally (still to be determined but I don’t believe it could be a 2.56)?
AND
3) Is there a code on the housing that could add to the story?

Just curious about the possibilities. Unless someone did some serious re-gearing in the TH400 transmission (is that even possible/all that likely) I don’t think there’s any way that R2 code is accurate.

Thanks for any input anyone can provide!

John

joe_padavano

The housings can accommodate any of the available gear ratios. The center section would have needed to be changed. 2.56 and 2.73 gears used a 2-series center section. 3.08 and 3.23s used a 3-series. All others used a 4-series. Aftermarket gear sets have been available to install 4-series gear ratios on a 3-series center section, but not on a 2-series. It's entirely possible that the center section was changed. The only way to know for sure is to pull the rear cover and figure out the tooth count or find the stamped numbers on the ring gear. On this example ring gear, the "9A42" indicates the pinion is 9 teeth and the ring gear is 42 teeth. 42/9 = 4.66:1 ratio. "13A42" would indicate 42/13 = 3.23:1. Aftermarket gears may not have the "A" but will still have stampings for the pinion and ring gear tooth counts.

72442TwiceOwned

Incredible. Thanks very much for those details. You have obviously spent a lifetime enjoying Oldsmobiles and paying close attention.

The cover was off earlier this year to replace fluid and put a new seal on. I’m afraid the mystery will continue, at least for now.

As imperfect as my car is, it bugs me that the code implies something that it actually isn’t.

Fun71

That is the case any time a gear change is done.
If you want some 2.56 gears to restore it to original, I have a set available.

72442TwiceOwned

Hey, that’s awesome and thank you. I’m not sure what my plan is right now. The car came with a 2.73 gear per the window sticker and it’s my understanding the O rear end wasn’t used in 72. I do, however, plan to get it out on the road and go to some cruise-ins and shows at some point soon. Buying gas for a few “next-state over” kind of show with the current setup makes me shudder a bit.

Anyway, I’ll keep your offer in mind. Just to have it in the mix what would those cost me? I have some other things to get done on the car but those gears would save a good bit on gas.

Fun71

I think you need to calculate how much you would save in gas along with how much it would cost you to have the gears installed and see if it makes sense. A long, long time ago I thought about an overdrive transmission in my car but calculated the cost of the transmission vs. the difference in MPG and decided it was cheaper for me to keep driving with what I had.

72442TwiceOwned

I totally agree. I was just wondering what the cost of the gears themselves might be. I also wonder with the rebuild on my motor being about 37 years old (but still with 125 psi on 7 cylinders and 123 psi on one) how much more wear and tear it’s racking up at the higher rpm’s. Not overly concerned but it crossed my mind. Thanks for the advice.

monzaz

ok first off... seems 245 / 60 / 14 are actually 25.6 inch tall... right? not 24.5" tall as stated. or do you not have the 245 / 60/ 14?
and since there is NOT a 3.73 gear in the 12 bolt olds rear end you have ...EVER...lol.
You have 3.91
I come up with 3592 RPM with the rpm calculator I have.
336 x 3.91 x 70 mph divide all that by tire size 25.6 = 3592.3125
Just trying to help.
Jim
JD

72442TwiceOwned

Hey, Jim. Thanks for the feedback and I appreciate the help very much. I do have the 245 60r14. I think Joe had commented on the diameter further up somewhere and then someone pointed out that wasn’t with the weight on the tires (I think the terms they used were “nominal” and “rolling”). I’m guessing the tire would expand some and be somewhere in between at speed. Anyway, I came up with very similar numbers as yours. I calculated using 24.5 and 25.5 assuming a 3.42 and then used 25.5 with a 3.91 (pics below). That last one seems to be right in the sweet spot of what my tach says, assuming it’s reasonably accurate. I realize now 25.6 would have been the better number to use but 25.5 seemed to make it pretty close. Enough to satisfy my curiosity, anyway.

John

Koda

It's worth noting that 2.73*1.48≈3.91

In other words, are you sure you're not in second gear?

Jeff Stout

Mark the pinion and brake drum. Rotate pinion and count how many times it takes to make drum turn 1 full revolution. If pinion. Rotates 2 5 ti.es the. 2.56 if pinion rotates 3 and 1 quarter turns then 3.23 ratio

72442TwiceOwned

Interesting. No, not in second. There are clearly two shifts when turning from a light onto the on ramp and accelerating to interstate speed. Good question, though. You never know.

72442TwiceOwned

Thanks very much for the info. I’m going to check with the shop that changed the fluid and just see if they noted what it was (doubtful). If not, I’ll give this a try.

BackInTheGame

You can download a free GPS-based MPH app for your phone. Very Handy! I use DigiHUD when on my FZR1 whose tires and sprockets are non-OEM. On my bike, using DigiHUD, I've learned my tire/gearing combo produces about 7% "high" reading from OEM on the speedo.

72442TwiceOwned

Thanks for those tips! I’m sure I am reading faster on the speedo than actual. I’ve just downloaded DigiHUD. I’ll plan to get the car out this weekend (if not sooner) and get a good speed and rpm reading (hoping my factory tach is at least close - it’s never been out of the dash and I have no idea how accurate they are at 50+ years old).

72442TwiceOwned

I’m going to start a new post related to my transmission to separate the topics but I had a new discovery today with my transmission that affected previous comments here.

I’ll call the new post “Transmission in My 72 442”