How do I find out what ratio a rear end is?

I have a rear end that I pulled off a 53 Desoto. It was a V8 with overdrive car from what the guy told me. I dont see any tags on the rearend, so I thinking I have to open it up to see what ratio it is. When I open it up how do I find out the ratio?

 

Divide the pinion teeth into the ring teeth .Example 11 pinion teeth and 33 ring teeth = 3.00 ratio .11 goes in 33 three times...

 

You don't need to take it apart. Just mark one of the axle hubs - maybe a stud- with chalk and make a corresponding mark on the axle housing. Then make a chalk mark on the U-joint or differential output flange AND another mark lined up on the diffferential snout. Then turn the axle one full rotation and not how many times the U-joint turns for one full axle rotation. It it turns 3 1/2 times you have a 3.50 or thereabouts - easy.

 

you have to turn both axles one full revolution, or turn one axle two revolutions and hold the other one still.

 

what squirrel says. seems that's twice in two days I gave squirrel the ditto. smart guy.

 

That doesn't always work.

Case in point:
A guy I used to work with had a '75 Toyota Celica, with 5-spd OD ******, and I know for a fact that those cars came stock with 4.111:1 gears in the rear end, during those years.
We were working on the brakes and I told him about the same thing Weasel mentioned, believeing it to be true, at the time.
We had another friend hold the opposite wheel, while he climbed under the car and mark a spot on the rear U-joint and the front snout of the differential.
I slowly spun the tire until it had rotated one revolution.

...guess, what...

He counted just a bit over TWO turns of the U-joint.
By his logic (and that echoed by major automotive rags over the years), that would mean that car had a rear axle ratio of just a little over 2:1.
Sorry folks, that ain't gonna fly.

A rear axle ratio is just that, a RATIO.
A ratio is defined as the comparison of two numbers.
In this case, the number of ring gear teeth to the number of pinion gear teeth.
If you don't have any tags on that pumpkin, do some research and see if multiple axle ratios were actually built (production only) for the car that axle came off of.
If not, then you'll know right then, otherwise, you're pulling the third member out.

NOTE: Anyone with a later model GM rear end, those numbers are actually stamped into the edge of the ring gear. All you need to do is to divide those numbers and you'll have the gear ratio for that particular differential.




6narow

 

6inarow--see my post above...you need to understand what the differential does, it makes it so there can be a speed difference between the axles, so the car will go around corners easily. This also means that you can hold the pinion still, and if you turn one axle, the other will turn the opposite direction. And if you hold one axle still and turn the pinion, the other one will turn twice as fast. If you hold one axle still and turn the other axle one turn, the pinion will turn only half as far as it would with both axles turning one turn.

The ratio is just a number...it's the number of turns of the pinion gear per one complete turn of BOTH rear wheels.

So in the case of the 4.11 toyopet you mentioned, if the guy holding the other axle still had turned it too, then you would have seen just over 4 turns of the pinion.

 

What squirrel said WORKS EVERY TIME.

What squirrel said...
you have to turn both axles one full revolution, or turn one axle two revolutions and hold the other one still.
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Squirrel's way works as well ,but its easy to foul up if you do it wrong ,(thats how my dad used to verify ratios also) but what do you do if you come across a ring and pinion set in a box at a swap meet and the guy tells you its a 4.11 ? How do you mark a driveshaft and hold the wheel then ? I dont go by tags either.I had a center section from a 57 Ford that had a tag that read "3.10" and it had 11 pinion teeth and 39 ring teeth on it .That comes to a 3.54 ratio.I thought the guy would like know the fool proof way ,and also understand where the word " Ratio" was coming from...

 

Counting teeth and dividing is the sure way to verify the ratio. The other methods are easier in many cases, but their accuracy depends on a lot of factors, including slop in the rearend, being able to hold two things that are 6 feet away from eachother, etc.

 

...and that's where your logic is flawed.

It's a general misconception that the gear ratio of a differential is a comparison of rotation.
It's not.
Its the comparison of the number of teeth on each of those gears (i.e., pinion & ring gears).

Think about it.
The misconception is that for every turn of the wheel, the pinion will rotate a particular number of turns....thus, "...-to-ONE".
It doesn't make any sense that you'd have to turn the wheel more than one turn, otherwise how the ratio is stated is incorrect.

If doing what you're doing works for you, fine. You go right ahead and keep doing that, but its based on a misconception, and that misconception will eventually fail you.

Count the teeth. THAT is the correct interpretation of "gear ratio" and will always give you the correct figure.




6narow

 

The ratio IS the number of turns of the pinion gear to the number of turns of the ring gear. That's just a fact.

The problem you ran into was that you didn't take the differential into account. If you turn one axle one turn, and hold the other axle still, then the ring gear will only turn one half of a turn.

I don't have any misconceptions about it, but if you don't understand what all is going on, then I agree that it is best that you take them apart and count gear teeth.

 

Why would the spider gears affect the relationship between the ring and the pinion gears?
The ring and pinion gears work directly upon each other, not through the spiders.
All the spiders do is allow one axle shaft to rotate independantly from the other, when you're turning corners.
...and why would GM stamp those gear teeth numbers on their ring gears if the ratio was based on the number of rotations?

In general a "gear ratio" can be defined by rotation, but in the case of automotive drive gears, its based on the number of teeth each of those gears possess.

Count the gear teeth. That's the SURE way.



6narow

 

if you only turn one axle take it times 2.

 

It does not. That's the whole point.

The spider gears affect how far the ring gear turns relative to the wheels. You have to get two full wheel revolutions to get one full ring gear revolution. It does not matter whether each wheel turns once, or one wheel turns twice. You need two full wheel revolutions somewhere.

 

That would be true, if you're referring to the axle that's driven by the spiders, but the other axle is bolted directly to the ring gear (via the hub), which bolts directly to the wheel and the ring gear is driven, directly, off the pinion.
The idea of rotation is incorrect. Its the number of teeth.


6narow

 

...and if you don't believe me, maybe you'll believe these guys.


<TABLE cellSpacing=3 cellPadding=0 width=419 border=0><TBODY><TR><TD vAlign=top>[FONT=verdana, arial, geneva][SIZE=-2]Figure 14 The numerical ratio of the drive axle is the number of the teeth on the ring gear divided by the number of the teeth on the pinion gear.
[/SIZE][/FONT] </TD></TR></TBODY></TABLE>




6narow

 

Ok, you've lost me.

All the differentials I've seen have side gears that connect only to the axles. The spider gears connect to the differential case and the ring gear. Neither axle is connected directly to the ring gear.

 

again,,,, What Squirrel said, 5 times.

 

Your right Squirrel ,but alot of people get messed up understanding how a differential works.Some guys think that the driveshaft turns and the wheels turn together in any scenario eqaully.The only time they will is with a spool or welded or " poor mans posi ".Thats why they dont make corners well when locked.I understand perfectly what your saying ,but alot of these guys dont .I figured the guy that ask the original question on how to tell may run into this same deal ,and thats why i told him to count the teeth.You are correct completely ,and i know you know what you are talking about .You have helped me out plenty of times with tips /info and i consider you one of the most knowlagable people on here...

 

Non-posi:
Jack one wheel off the ground.
Turn the wheel 2 complete turns.
- Count the turns on the drive shaft.

For example:
3.55 turns on the drive shaft equals a rear ratio of 1:3.55


Posi:
Jack both wheels off the ground.
Turn one wheel 1 complete turn.
- Count the turns on the drive shaft.


Just gotta be accurate when turning and counting.

 

...and that's why I wrote "(via the hub)".
Maybe I should've written "(via the diffrenital case)".
Sorry if that was misleading and yes, you're correct. The ring gear does not bolt directly to the axle.

But we seem to getting away from the point a little.

In this case, "gear ratio" is the comparison of the number of teeth that are machined into the pinion and the ring gear.


6narow

 

Hey, we're trying to make things easy here (see my signature) and RoadRunner has done just that - thanks. This method has always worked for me. I should have posted two turns of the axle/wheel for non posi - my bad.

I am not sure that **Welder** was looking for a physical science lesson when he posted - just wanted to figure out the ratio and RoadRunner has provided the simplest way.

Otherwise I'm with Squirrel, but then I would be wouldn't I - us rodents have to stick together.

 

Marked the wheel and axle and had my wife hold the other to keep it still. Turned the wheel twice. Blamm.....its a 3.55 rear. Just like the guy said.

Thanks for the science course on rear ends.

 

What the....


"will eventually fail you" ?? wtf x2

non-posi: turn the wheel 2 exact times and it will never fail.

....and "comparing rotation" is the same dam thing as comparing tooth counts...

 

**WELDER**, I have a book with the rearend axle ratio's for a 54 Dodge. It might be different than the Desoto but for an OD car they had 4.11 and 4.30 as an option.


Did you have any luck with the 3.54 axle I sold you?

 

Gosh, I've got the perfect signature !!
squirrel right, 6narow wrong.

 

I've been involved with gearing as a profession for over 40 years. I worked for the largest gear machine manufacturer in cutting, grinding, lapping, heat treatment, R&D, machine installation at the customer's plant, training customer's personnel and troubleshooting ***embly problems for rear axle manufacturers all over the world. Trust me, Squirrel is exactly correct in his directions on checking the gear ratio by rotating the axle, to give a very close idea of R&P gear ratio. It can't change, it's math - i.e. 41 ÷ 11 = 3.72272 or about 2-3/4 turns of the u-joint - ALWAYS !

If you want the exact ratio, count the number of teeth on the pinion & ring gear and divide the number of teeth on the gear by the number of teeth on the pinion. It's pretty simple math formula and it won't change from axle to axle.

 

¡Excellente!

Simple does it best... and with the least amount of effort even better!

Never the less - I learned a lot from really knowledgeable and selfless folks on this site who often explain things in great detail.
Everybody wins in the end.
Thanks everyone.

 

DO NOT SPIN THE TIRES. You will not get an accurate reading, whether open rearend or posi. There is too much slippage by spinning a tire.

Jack it up, mark a tire, spin the pinion or driveshaft. Count the number of times the pinion spins for one revolution of the tires.