Ken described a rough method to do this in chat last night.. and my GrandFather had shown me this once before. (unfortunately i have forgotten more than I now know)

I thought about it and figured out that there are too many ratios available that are close. ie 3.55/3.70 and there are custom ratios that i dont think were ever offered in our vettes to rely on a rough estimation. for those of us who arent the FO and have no idea what any PO's did.. you want to be able to verify gears without a major hassle.. so i took the ones I could find info on and figured out the marking necessary to find them..

all these calcs are based on my measured drive shaft circumference which was 22.65cm...

if you jack up the rear end of your car.. (on stands of course) and put trans in neutral. then rotate your right rear (so this will work for posi/non-posi) tire until the valve stem is straight down..

then make a crayon mark on the drive shaft (long ways) at this reference mark in one color (yellow in my case)

then i used a small piece of elec tape to attach a sewing measuring tape.. and i rotated the shaft clockwise and marked the known offsets i thought were most likely (i was pretty confident about which one and only marked 3.08,3.36,3.55 and 3.70) which were 1.8,8.2,12.5, and 15.9cm respectively

then recentered the right rear tire, got back under the car.. had someone turn the tire one full turn, i counted full rotations (3) of my yellow mark and then waited until it stopped on one of my red marks.. which was the 3.36 mark.

and i verified the ratio in 10 minutes.. (plus the half hour to build the spreadsheet and research)

im working on a printable cutout that you can tape on your drive shaft to do this easily..

can someone verify that the drive shaft is 22.65 on their 68-79 and tell me if 80-82 has the same size shaft also..

also in my spreadsheet i listed all the ring/pinion gear counts i could find.. id be happy if someone could look it over and tell me any others

http://www.direclynx.net/~cthulhu/corvette/charts.html

The only thing I would like to add is you must turn BOTH rear tires 1 complete turn. If you have a posi this done automatically.

If you don't have a posi, either lift both tires, mark them both to ensure exactly 1 turn on each. OR only lift only one tire and without turning the other side at all, turn the one free tire 2 turns. This works due the the function of the diff.

i fixed the chart so it should work now

also I found some neat info..

never thought about this.. but here is an excerpt from a web page that explains why most ring/pinion combos have at least one prime number..

Both gear sets listed above have one gear with a prime number (11 and 37). Those prime numbers are what gave us the messy math, but the prime numbers also have an interesting effect on the gears as they run. It causes every indvidual tooth on the pinion to mate with every tooth of the ring gear. In the case of our 4.09:1 ratio, every 11 rotations of the ring gear sees each tooth of the ring gear be touched by each tooth of the pinion. What this does is promote a very even wear pattern on the gear teeth, which greatly extends the life of the gears.

|UPDATED|6/4/2004 9:17:56 PM (AZT)|/UPDATED|

You gotta love the work ethic and dedication both of you gentlemen bring to this site. I speak for everyone when I say...THANK YOU!

JR

i still need someone to verify the diameter of a 68-79 drive shaft and the drive shaft of 80-82

then ill post my printable cutout that you can use to id your gears..

just measured the drive shaft od on my 79. it's 2.5". hope this helps.
Hank

hank51 said: just measured the drive shaft od on my 79. it's 2.5". hope this helps. Hank

can you measure circumfence.. this is a much more accurate way to measure

unless you measured with calipers ???

I used calipers, but if the od is 2.50", then the circumference is 7.854" (2Pi x R)
Hank

thanks

this bugs me now as the circumference of your drive shaft is smaller than mine...

which means now i have to deal with different drive shaft sizes in my calculations

if you want, email me your data and maybe I can help you organize it or suggest a way to that you haven't thought of.
Hank