Rearend ratio???

Can one figure out the rearend ratio without pulling the cover and actually counting the teeth on the ring and pinion???

My rearend is from a '64 or '65 Nova and the trans is a 400 turbo. I have 235/70R15's for tires and when running down the road at 60 mph the tach is right at 2500 rpm. With that combination can the ratio be figured?? Anyone good at Math??? I'm not.

 

You can get close by jacking it up and putting the car in neutral. Have one tire blocked so it won't move if you have an open rear diff, and spin the other wheel 1 full turn and see how many times the drive shaft makes a full turn...

 

If you have a posi, you should not block the other wheel.

 

There is a formula that you can use but you are going to be close at best. Tires although close don't all have the same diameter.

You can put the car up on a jack and turn the wheel if it is a posi it will be easy but if it is open you may have to block on wheel from turning. Count how many revolutions the drive shaft makes for one revolution fo the wheel. You will be more accurate if you turn the wheel twice and devide by two.

Or you may get lucky and find that the ID tag is still intact. read what the tag says to get exact rear gear.

 

3.47:1

Bill

 

Thanks for the replies. There is a tag on it but it is in a position where I can't see it. I knew about the wheel turning thing but thought it could be figured another way. 3.47:1 is close to a 3.55:1 so that may be what it is. Think y'all are right about turning the wheel and counting the drive shaft turns being the best way.

 

3.55:1 is a pretty common ratio. One thing that should be kept in mind is that tire diameter is a nominal number and is not only affected by manufacturer but also by wheel width. The formula does not take into account tire growth due to rotation.

If someone is willing to use the formula to determine the ratio it would be helpful for you to run a tape on the tire mounted on the vehicle. That will get you a little closer.

 

http://www.richmondgear.com/101032.html

Good link to figure this stuff out.

 

Better yet, grab a screwdriver, flashlight and a wire brush...can of carb cleaner/brakekleen optional.
Slide your *** under the car from the back and clean the tops of the axle tubes [usually the driver's side tube but not always] to find the 2 or 3 digit letter code stamped in the tube-top. . PM me with the numbers/letters and I can cross reference the code in my old Hollander manuals to give you exactly what it is.

 

Thanks for the link Boryca. I'll have to go out and measure the tire height and see what that comes up with.

 

I done that, but can't get my head high enough. Used a mirror and wrote down the numbers. Found the piece of paper I used and I have axle then some numbers but more than two or three. How bout ......BA60C...I think that is the one I got from the top of the axle but will take another look when I get a chance.

 

http://www.sccoa.com/faq/tirecalc.html

Here's a link to calculate the tire height based on the other specs. Yours are roughly 28".

 

3.55's sound about right. Plugging in your tire size at 28" tall at 60 and I come up with 2556 rpm

 

I've been out in the garage but forgot to measure the tire height. But calculating at a tire height of 27.95 which I get from the tire calculation link, and I get 3.47 .....so V8muscle...You were right on. So it's probably a 3.55 but when I get a chance I will raise up the rear end and spin the tire and driveshaft twice around and see what it gets that way.

Thanks guys for all the input.

 

You need to turn the jacked up wheel TWICE and count the turns of the driveshaft to get the ratio.

Here's a short video I made that may help.

http://www.youtube.com/watch?v=bkvp3Fq21ZM

Mart.

 

64-7 nova rear codes ba=308 bc=336 bh=355 posie be=308 bf=336 bg=355 br=307 bs=331 bu=307 bw=331 fk=355 12 bolt fl=355 fo=373 fp=373

 

I was gonna guess 3.08 and that BA code seems to confirm it?

but the turn-the-driveshaft trick works good. Make sure to take up the slack before you start counting.

 

i have used this on later model rears,but how far back in years does it go? didn'tknow it was on older stuff thanks for info on this.

 

You can find lots of identification codes in the "info kits" that Chevy has available for download.

http://www.gmheritagecenter.com/gm-heritage-archive/vehicle-information-kits.html

Also in the old Chevy parts books...but they usually cost money

 

Exactly.
Used to do it ALL the time when I was checking Ford junkers for suitable 9" centersections.
With a non-Posi, make sure that other wheel isn't turning or you'll mess up your count!

Many just spin the wheel once and DOUBLE the driveshaft revolutions to get the ratio...but for someone not used to the procedure its easier to just spin the wheel twice and not trust your brain! LoL

 

Well, I finally remembered to do the driveshaft thing. Turned the rear wheel two turns and the drive shaft went around just a tad bit over 3 turns. Say like the dial of a clock, it would be about 1:00 past the 3 turn mark. so what y'all think, 3.08 or 3.55. I think it's a 3.08, being just a little over 3 but not close to half

 

yup, that's 3.08

 

24 hours from start to finish.....I'd have removed the cover in half an hour, but that's just me.

 

I"d have jacked up the wheel and turned it and counted in about 3 minutes, but that's just me.

 

24 hours to do what??????

 

Your at the s****yard checking 9" Fords.
15 of them.
Still feel like pulling all those axles and centers so you can count teeth?
Do you honestly think the yard owner is gonna let you pull 15 rearends apart and look???? Seriously????

A single rearend I can check the ratio in MINUTES if I have a jack or the rearend is out of the car.

 

He's talking about from the time I made this post yesterday morning. Been too busy to get right to it but got it done today.

 

....and what does a group of 15 9 inch Ford rears with removable center sections have to do with a single Chevy II rear end that has an easily removable rear inspection cover?

 

I'd probably want to pull the gears out and inspect/replace the bearings, inspect the pinion gear carefully for cracks, and check out the axles for wear, too.....but turning and counting is a quick way to figure out what you have.

 

It's called an "example" to show why its nice to know the easy way to check ratios.
Could be 15 rearends with covers if that works better for you.

Regardless...I'd STILL have 15 ratios checked before you had two done!

Then, once I knew which rearends had the desired ge****ts, I'd pull JUST those covers to check for internal problems.

Why do you have an issue with this guy (and anyone else who might read this) learning to check ratios quickly?
If you want to pull things apart to do a simple check then go for it...but we're offering a simpler way.

Read his first post...he's REQUESTING a way to check WITHOUT removing the cover.

He's just wanting the ratio so he can do some RPM math towards better highway manners!

Don't be such a HEATHEN! Hahaha