i have a 66 elcamino with a 12 bolt rear, how do i determine the gear ratio without removing the diff cover

Might try checking to see what is stamped on it to see what it was originally.

http://www.chevelles.com/years/66/66rearcodes.htm

That picture might be wrong though, I don't know about 66 but 70-72 the stamping was on the front of the axle tube, not on the rear like the picture shows.

The last one I checked was stamped for 3.31: Posi but when I pulled the cover I found it had been changed to a 4.10: Posi.

If you have a tach, another method is to use one of the online gear/speed calculators like http://www.angelfire.com/fl/procrastination/rear.html . You'll need to know your tire's diameter. Drive the car in high gear (1:1) and note the rpm at a given speed. Enter the info and it'll tell you what your ratio is. If you have an auto you may not get an exact match due to slippage but it should get you close. Like Dean posted, codes on a 40 year old differential may not be reliable as gear swaps are not unusual.

Jack the rear end up, put a small piece of masking tape on a certain spot on the rear of the driveshaft. Turn the rear wheel exactly one full turn while watching how many times the tape on the driveshaft turns. If the driveshaft turns about 3 1/3 times for one wheel turn - then 3.31 ratio. If it turns 3 1/2 times - then 3.55 ratio. If it turns 3 3/4 times - then 3.73 ratio. Etc., etc.

I may be wrong but I seem to remember that there is a formula where you could raise it up and calculate it buy counting how many turns of the drive shaft buy turning the wheel one turn. Maby someone knows if this is so or not.

Uh OK,Don that works perfect!!!!!