Trying to calculate my rear end gear ratio and want to make sure I've done this correctly. I counted the teeth on the ring and pinion and they came to:

Ring: 41
Pinion: 16

From what I remember, to calculate the gear you divided 41/16 which equals 2.56. Is this correct? I have a 2.56 rear gear?

Thanks for the help


Robert

Are you sure it's 16 on the pinion and not 15?

I was pretty sure it was 16. I'll double check it in a couple of days when I'm out fitting the new gasket.

2.56 was available in some GM rear ends, don't think I've ever run across it in a 7½" though.

If it's a factory gear, the tooth count will be stamped on the edge of the ring gear.

15 and 41 is the extremely common 2.73. That's more likely what you have.

i maybe wrong but i believe the teeth count on either gear will always be a prime number, and 15 and 16 aren't prime.

Axle Ratio Tooth Combination
2.56:1 41,16
2.73:1 41,15
3.07:1 43,14
3.08:1 40,13
3.08:1 37,12
3.31:1 43,13
3.36:1 37,11
3.42:1 41,12
3.55:1 39,11
3.73:1 41,11
4.10:1 41,10
4.11:1 37,9


Granted from a 2nd gen site but it should work .... (but they where 8.5" rears)

Not sure if it would be different for a 7.5 or 7 5/8

http://www.nastyz28.com/camaro/zsusp.html#gearteeth

that is the site above

I think it's desirable to have them prime, but they don't have to be.

In sensibly designed rear ends ( NOT Brand F for example) at least one of the numbers is usually prime, but not always. Some common examples are 3.42 (12 & 41), 3.08 (12 & 39), 3.23 (13 & 42), 2.73 (15 & 41), etc. A few of the common combos are 2 prime numbers, like 3.73 (11 & 41), 3.36 (11 & 37), 3.31 (13 & 43), etc.

They should at least be relatively prime, so that the greatest common factor between them is 1. That way every ring tooth will at some point come into contact with every pinion tooth during operation.

Ok, went out today and recounted. Turns out that it is 15 (also had the number stamped into the ring gear) and therefore 2.73, which is what it came with from the factory according to the RPO codes.

Now, does anyone know why my speedometer is reading about 25% UNDER my actual speed? Checked it with 2 firends pacing me. Their 50mph was about 40 on my speedo and their 100mph was about 80 on mine.

Only thing I can think of is that the speedo gears were swapped by a previous owner

Suggestions please.


Robert

My 'guess' would be the tranny was replaced and the speedo gears weren't swapped or you aren't running the stock height tire(taller).

Unless the tires are 25% taller than stock it's more likely the transmission has been changed.

Wheels are the stock 16 inch IROC items.

So it's most likley the tranny has been swapped and whoever did the job didn't swap over the speedo gears that were in the replacement?

Can someone tell me of a place where I can get the speedo gears online? I remember seeing a place that did them and they had a table listing all of the combinations, anyone know the webpage?

Also, anyone have any pictures of the procedures for removing the two gears? I read the tech article, but a picture would be nice.

Thanks again for the help guys. It's really appreciated.

Robert

Try <A HREF=http://members.shaw.ca/crussel/speedo.exe>this</A>.