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How do you figure out the rear gear ratio if it is not what the axle code numbers show? Does the gear ring have it stamped on or do you have count the teeth and use a formula?

A friend said his rear axle code decodes as a 2.56 gear posi (it's a 12-bolt). This isn't what he is running because he tachs over 3200 rpms when going 70mph. The trans is a TH350. Plus, I don't think 2.56 gears were put into 12-bolt posi rears. The rear is out of a '69 El Camino.

Can you guys tell us how to figure the gear ratio? Thanks a lot.
 

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If you pull diff cover and count pinion and ring gear teeth. Divide # of ring gear teeth by the # of pinion teeth. For example my 12 bolt has 43 ring gear teeth, and 14 pinion gear teeth. So, 43 / 14 = 3.07142857142857142857142857142857 (rounded to 3.07:1).

Rear Numerical Codes, shows 2.56 was availible in 70 at least.
http://www.chevelles.com/years/70/70rearcodes.html

DG



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DG
Springfield, Ohio
70 Chevelle Malibu

www.wright.edu/~s001dga/chevy.htm

TEAM Chevelle Member #0086

 

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My truck manual has a tire speed chart that shows if you have a P225/70R15 tire at 70 mph you will have the following rpm.
3:55 rear=3160rpm
3:73 rear= 3320rpm
4:10 rear=3650rpm
If your rear tires are close to this size this might be close to determining your ratio without removing the cover.
 

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Also, if you measure your tire dia and go to tech reference on this site you can run a simple equation and get your answer.

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You can jack the car up put marks on the drive shaft and tire.

Then rotate the tire and count the number of drive shaft rotations for one tire rotation.

It is not as exact as the dividing the number of pinion gear teeth into the number of ring gear teeth but you don't have to remove the diffential cover to do it.

If one tire rotation produced a little over three drive shaft rotations then you probably have a 307:1 or 308:1 gear set. You could further mark the drive shaft at 45 degree incriments all the way through 360 degrees and calculate a little closer.

Then it is probably a good excuse to remove the differential cover drain the old nasty oil, clean all the metal filings out and add nice new gear lube after you do the math and have the exact answer.

John
 

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There are axle ratio codes( two or three letters and a few nubers ) stamped on the passanger side axle tube, halfway between the diff and wheels facing the front of the car. This will tell you the original ratios for that rear end and if it was posi etc. If not stock, then refer to the other posts. I have the codes from 69 up and some are posted on the tech archives.
From your buddys cruse rpm, sounds like at least 3.08 which is what I have

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I hope someone can correct what I'm doing wrong. Per the discussion, I jacked up my car, made my crayon marks, and spun the left rear wheel one revolution. The driveshaft spun roughly 1 and two-thirds (not quite 1.75) times. It doesn't sound like a legitimate gear ratio to me.
Does the other tire need to free-wheel or be stopped. The right tire remained on the ground. Additional info - it's a 12-bolt and it was sold to me as having 4.11 posi but I may have gotten stuck... Thanks..... TK
 

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TK, all I know is my posi won't spin one wheel when the other is on the ground. I just finished brakes and such and I noticed that. If you run a search on rear end gear ratios somebody, just recently, posted the right way to do it. Without that info, I would simply put jack stands under the axle,so both wheels are free.

I hope I'm wrong, but it sounds like you got jammed on the posi.

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[This message has been edited by Gene Chas (edited 06-15-99).]
 

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Gene, thanks for the response. Boy am I bummed... First I discover the "M21" in the car I just bought is evidently an "M20", and now the posi/4.11 thing doesn't sound promising.
Can someone say for sure how I can validate this? Please don't take offense Gene, you just seemed hesitant on your reply. I know if I spin one wheel, the other spins in the same direction. However, I can also spin one with the other on the ground as well.
Maybe I'm losin' it, I dunno... Any help is much appreciated.. I need something to go my way on this car. Thanks, TK
 

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TK Based on the info provided and many years
of experiance. Pull the cover. That rear is
old, possably abused and you need to be sure
of what you have.No matter what condition it
is in you will feel better if all you do is
change the oil. While you are in there pull
the C-clips and check the axels at the
bearing surface. check gears for galling,
missing or damaged teeth. Loseing a rear
wheel while in motion, is a real mess, Tire,
wheel shot, unusal costomizing of the quarter
panel yours and possably the guy next to you.
which could result in a a$$ kicking on top
of all the damage. Hardly worth it, for the
price of a gasket and some oil.
Harley
 

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When only one wheel is raised, it must turn twice to equal one turn for both wheels. To get more accuracy, you could count more revolutions.
Try turning one wheel 20 times counting the drive shaft turns. Divide the result by 10. 30.7 drive shaft revolutions would equal a 3.07 ratio.
 

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Gents, thanks for the advice. You may see my car in the classifieds soon. This is just one in a long string of surprises. I searched and searched for my dream car, and I did something that I've never done before - I bought the car on faith. I trusted the seller. I was shown written appraisals from a few years prior documenting M21, 4.11 posi, etc. I believed it was all true because it was in writing. And now I'm paying the price, surprise after surprise... TK
 

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TK,
Try not to feel bad. When I bought my 69 SS I received copies of every receipt on the car totaling 25,000 dollars. The engine was rebuilt 20,000 miles ago, along with the trans, paint etc. etc. He even gave me his log book showing the mileage and what was done to the car throughout the 15 years he owned it. I truly believe he did everything he had written down in his logs.
Well on Friday I will be back on the road again. I developed an oil consumption problem. (Like 1 quart per 100 miles) Oil pressure was at 40 to 50 lbs. Also the transmission would start to grind after a long trip.
I now know what I have and all though I didn't want to spend this kind of money at this time I'm sure looking forward to getting back behind the wheel. So take a step back and form a plan of attack to make a good cruiser. Summer time is here.
Tom....
 
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