442b

Trying to figure out what gears I have in my 72 442. Someone told me I had 342's but I think they are higher. Is there a way of telling by what the motor tachs? My motor tachs 3000rpm at 60mph. Any guesses? W-Machine, do you have any idea? Thanks

MDchanic

Why not just count the revolutions?

Jack up the rear, mark one wheel, rotate the driveshaft until the wheel has gone around exactly once (easier with a helper to watch), and count the number of revolutions of the driveshaft it took to get there. If you've got a 3.42:1, it should take just under 3 1/2 turns to turn the wheel once.

- Eric

442b

Hmmmm, Ok. I guess I'll give that a try. Didn't know that could be done, I'm more Machanically Declined than Inclined. I'll post what I get & maybe you can figure it out for me. Thanks

MDchanic

Make sure both wheels are off the ground - if one can't turn, the other will turn twice as much.

And mark your starting and end points with chalk, because it's easy to forget.

- Eric

442b

Will do

1969w3155

Pull the differential cover and count the teeth, that way there is no guess work. It's an easy job.

MDchanic

Yes, but that involves getting involved with gear oil, which is disgusting,
and cleaning and re-sealing the gasket surfaces.

Also, unless your rear can be cleaned perfectly , it involves putting in new oil, and a measured amount of friction modifier if it's a posi.

- Eric

arodenhiser

I have always jacked up the rear end so both wheels are off the ground. Then mark the driveshaft, masking tape works well, and spin the one wheel ten rotations. This seems like more work, but sometimes it is hard to tell the differnce between similar ratios. Counting 3.08, 3.28, 3.36, or 3.42 rotations of the driveshaft is very hard if just do one tire rotation. When you do 10 tire rotations it is easy to count 30.8, 32.8, 33.6, or 34.2 rotations on the drive shaft. A helper to count or spin the tire are a big help.

Adam

MDchanic

Good point, Adam.

I've only done it with Chevys, where it's pretty much only 2.73, 3.08, 3.31, or 4.11, so it was easier to see the difference, but with all those similar possibilities, ten revs is definitely a better idea.

- Eric

1969w3155

You make it sound hard , which it isn't. It's gravy.

2blu442

If you knew the tire height with rpm at a specific speed there should be a way to calculate the ratio. And if it's not been messed with there should be (but sometimes is hard to find) a two letter code stamped in the passengerside axle tube of the factory gear ratio.

But... I'm with 1969w3155 on this one. When's the last time that gear oil was changed? This would be a good way to know for sure what the ratio is, does it have a limited slip or not, is there metal shavings in the oil, or is it just plain dirty and in need of fresh oil! My 2 cents worth

John

64Rocket

Why don't you clean the passenger side axle tube and get the code and check shop manual.
Olds code for 3.42 is "TM" or "S9"
3.90 could be "TO" or "T3"
3.23 could be "SF" or "S7"
Gene

MDchanic

It's not so bad if you've got to change the oil anyway, but it's a big pain compared to lying under the car for five minutes turning the driveshaft.

Maybe it's just me. I hate messing with gear oil.
Everyone's got something they can't stand - for me it's gear oil and exhaust systems.
I'll pull the whole dash apart and put it back together to make a minor wiring change. Some people won't go near electrical stuff, and would rather live with a light out, or an annoying warning light or buzzer that's not disabled. Me, I'll avoid opening the rear unless I have to.

- Eric

A72CUTLAS

Where exactly on the axle would I find the code. I had looked for it before(sorta) and didn't find it. Is it on the top, side, bottom? Thanks for any info.
Walt

stevengerard

usually on the side facing the front if I remember and when looking at it form underneath it will be upside down, someone can correct me if I'm mistaken, been a while since I looked

507OLDS

If you are spinning 3,000rpm at 60mph,my guess is you have 3:73's in the rear,maybe 3:90's.

2blu442

The location of the code can vary, and I've had a couple housings that I never found the code on. Here's two examples,

P1010128.jpg
This is a type O rear end with the R2 code stamped upside down on the front, near the breather and brake line.





P1010129.jpg
I think this one is a corporate housing like what you should have, and this one was on the front/bottom. This one is a code SA. I don't know what the B089 1 means, but its the two digits of letters or a combination of one number and one letter that specifies the gear ratio. John

442b

Well, I just got finished doing the spin thing. Counted drive shaft rotations. Did it three times just to be sure. The drive shaft spun 3.2 times so I figure it's a 3.42 rear. It has the posi tag bolted to the rear cover which I removed & cleaned it up a bit just to see if there were numbers but it just has instructions on it to use Limited Slip Gear Oil Only. My nephew & I are going to attempt to remove the fuel tank soon, one of the rubber lines has a slight leak so we'll replace it with a new 6" rubber fuel hose. The reason we're going to remove the tank is to hopefully find the build sheet which would be awesome. A friend of mine has a 71 Cutlass that he replace his fuel tank on & when he got it down from the car, there was the build sheet, stuck to the upper side of the tank, pretty cool......

So back to the rear gear, Am I correct in assuming the rear gears are 3.42's ?? Thanks for the help guys & great information you all bring to the table, What a Super Group to be part of. Brian

2blu442

It could also be a 3.23 ratio. If you can find the code stamped in the housing, the 3.23 would be SR if it's a non limited slip or SS if it is. The next taller ratio would be a 3.08 code SC open, SD limit slip.
John

mmurphy77

70-72 442s used 3.23s (3.42s and 3.90s were optional I believe) so if you counted 3.2 then I would assume you have a 3.23:1 ratio (which is what I have in my '71 442).

coltsneckbob

I have a 1970 with the 3.23 posi. The stamped code is on the axle tube on the passenger side. At least for my car it is on the rear side of tube (facing gas tank) and is about mid-way from diff housing to brake shield. It is upside right. The axle is a posi and for my year the code is SF. I don't know if SF is correct for a 72 3.23 posi.

442b

Ok, then if I'm doing 60mph on the highway and I'm taching 3000rpm, it seams it's a higher gear than a 3.42. And if I had 3.23 which should be lower, I'd assume at 60mph, I would be taching lower like maybe 2000-2500rmp. I'll get my nephew to crawl back under her next weekend & see if we can find more numbers. Today was just a quick peak. I'll update this thread when I have more info. Keep up the convo guys, it's great learning from all of you, whether right or wrong, gaining knowledge about our Oldsmobiles is so much fun.

coltsneckbob

Are you sure you are going 60MPH? Are the tires the correct size for the car and speedo? Is the speedo gear in trans correct - for the tires u do have?

Also, while I've not paid that much attention to tach and speed ratio on my car I do have the sense that at 60MPH I would be close to 3K rpm.....although I am not real sure about that. I do know that the rpm are generally considerably higher when I compare it to my newer cars with 6 and 7 speed auto-trans.

I have also heard that the tach in these Olds sometimes gets out of whack and will show a higher rpm than actual. Have you compared your tach to a separate meter? You may need to calibrate the tach.

442b

Good questions. The wheels are stock 14" with BF Goodrich tires. Speedometer should be correct, (tested in speed zone using police radar). Tach is aftermarket non oldsmobile & that should be correct also. As far as it being hooked up correctly, I can only assume it is. My car is a 4 speed if you were wondering that or if it makes a difference. What do you think?

coltsneckbob

If you tested the speedometer against police radar (I assume that was a friendly test and not a ticket ) then you can eliminate that as an issue.

If the tach is hooked up properly - and if it weren't you'd probably get no reading - then I suppose your reading is correct.


The 4 speed should not be an issue - well unless ur forgetting to shift into 4th and running at 60 in 3rd gear LOL - but I kinda doubt that.


As you can tell from 2blu442's pics the stampings are sometimes hard to see even on a clean tube. You may have to search carefully to see them and might even have to wire brush the tube to make it out.

As I said before though - I have a feeling that even with a 3.23 rear end u might tach up to 3K at 60 mph......if the weather here weren't so crummy and if I didn't have a cold I would take my 442 out to see what the tach is at 60.....but perhaps another time !!

442b

Sorry to hear you have a cold. Get well soon Bob. Yes, the police speed limit test was friendly. One of those side of the road flashing your speed at you as you zip by at 90, LOL..... Joking. Anyway, if you get a chance to check your cars tach at 60mph sometime, that would help but I plan to check the tube in a week or so. I'm sure we'll find something. Thanks again

Bunser

After reading all the comments, I would suggest you have a 3.91 rear axle ratio.At 60 MPH my Cutlass would read 3000 RPM.

70 cutlass s

Guess no more enter your RPM, speed, and tire diameter. Find out in about two seconds.

http://www.csgnetwork.com/gearratcalc.html

507OLDS

If that is a real 72 W30 4-spd,then you probably won't find a sheet above the tank,as the Lansing cars rarely had them up there,and most didn't have any papers at all.The 72 W30 4-spds came with an M20 wide ratio 4-spd.It should have come with a 3:42 or 3:73 rear,unless ordered differently.If you look at picture #2,for a reference,that is where your code will be located on a 71-72 rear.The first 2 letters are the ratio code,then one letter for the plant,O for Oldsmobile,or B for Buick.The next numbers,like the 089 are the date it was finished,the 89th day of the year.

BlackGold

As Bob pointed out, you can't calculate your rear end ratio using speedometer and tachometer unless you know that both are accurate (and you know your tire size). Factory tachs are seldom accurate.

442b

Well, we changed the gear oil today 10/10/10 , Replaced the old, extremely dirty oil with fresh Valvoline 75W90 Synthetic Gear Oil. Cleaned the cover nice & gave it a fresh paint job. Read some numbers off the Ring Gear (41412GM CL2512). One of the other gears had in Yellow paint the numbers either 100 or 001, however you want to read them. Cleaned up the rear Tube & found the following on the passenger side, facing the front: TM C348. Not sure what they mean. I believe the TM = 3.42 Gears. Not sure what the C348 means. May one of you gents could decifer furhter. And before anyone asks, NO we didn't count teeth even though we were in there, Totally forgot about doing that. And NO didn't do the tire circumference thing even though tires were all off the ground. Again, totally forgot about doing that. Anyway, Thanks for all your comments on this subject, I'm convinced I have 3.42 gears even though tacking 3K @ 60mph seems awefully high. Brian

2blu442

Brian, I've got a 3.42 rear and it has 41 teeth on the ring gear with 12 teeth on the pinion. I would have expected there to be spaces between the numbers, but your 41 41 12 GM could be designating the 41 and 12 teeth. I think you nailed it! John

507OLDS

The TM is a 3:42 posi,and it sounds like you have the original GM gears in it,so they should be the GM 3:42's.The "C" is a poorly stamped "O",and it was built the 348th day of the year.When was your car built,late-January,early-February/72?

442b

Not sure when it was built, how would I find that out? I do have the protecto plate and original paper work from the dealer but no Build sheet. I pulled the back seat out today also but wasn't able to find anything. Decided not to pull the gas tank down until I use up most of the fuel in it. I have a fuel hose with a minor leak, very small seeping so we're going to replace it, just need to have less fuel in tank. Someone in this thread said they didn't think the build sheet would be on top of the tank because the car was built in Lansing. We'll see. One of these days we'll pull it down. Thanks for any info you have to share. Brian

507OLDS

The build date is on the left side of the cowl tag,with your trim codes.It will be numbered 01-12,for the month,then a letter A,B,C,D,or E,for the week of the month.
Example: 02C = 3rd week of February.

442b

I'll get it in the next couple days & post it here for all. Thanks

Bikedude

Your'e pic looks like NRP! BTW, my 69 442 turns 3K @ 60 and I have 3:55's in her.

507OLDS

Yes,Norwalk,now called Summit Motorsports Park.
I have 3:73's in my 72,and it turns 2800rpm at 55mph,with a 28" tall tire.

Chesrown 67 OAI

If you have the PROTECTO PLATE - you dont need to find a build sheet. All the info you will ever need to know is in the code numbers and letters of the P-O-P!!!
Someone here can decipher it for you! I'm sure 507Olds(Brian) can do it for you!!

Chesrown 67 OAI

yup -- my sig pic is at Norwalk too!