Rear end
Drifting
Joined: Apr 2008
Posts: 1,305
Likes: 2
From: Lewisburg WV
thegazman
Wow! I don't know about 1970, but in 79 there were several gear ratio options available. I'm sure someone will chime in here with the differential codes that will tell what your ratio is. If you can't find the code on the differential you can jack the rear end up, mark the driveshaft with chalk, turn your rear tire and count the number of times the driveshaft turns in relation to one turn of your rear tire. I have a 3:73 in my 79 and the driveshaft turns approximately 3 3/4 turn per one turn of the rear tire. Also, there are some sites where you can type in your speed and RPM's to get your gear ratio. Example: On my car 70 mph gives me close to 3500 rpms. Great around town, but a pain on a trip.
Platinum Supporting Vendor
Joined: Sep 2010
Posts: 533
Likes: 2
On the bottom of the differential is a stamped code. In 1970 there are 18 possibilities. The three letter prefix (caz, cfb, ...) should start with a "C". If you can find that we can determine which ratio you have.
Tom
1-800-962-9632 ext-240
Tom
1-800-962-9632 ext-240
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Race Director
Joined: Feb 2006
Posts: 12,239
Likes: 19
From: Chino CA
Safety Car
Joined: Sep 2007
Posts: 4,458
Likes: 330
From: Indy Indiana
expecting that the gears are original on a 40 year old car may prove to be expecting too much. you can look at the code and it will tell you what the case was originally built with, but there is no guarantee that that is what the gears you have in it, nor that that pumpkin case is the original either.
your only absolute way is to dissassemble and count the teeth. you can of course get under it and spin the driveshaft and count the wheel rotations. that should get you real close.
you can also do the math with the tire diameter, exact engine rpm and using a GPS exact speed. but this introduces error in speedometer, tachometer, and tire size that you may measure. take several readings and average it out and you should get real close.
the are also several calculators in the internet that will get you real close.
here is an example of a 12:37 ratio stamped on the pinion gear with the date.. 12/37 = 3.08 ratio.
your only absolute way is to dissassemble and count the teeth. you can of course get under it and spin the driveshaft and count the wheel rotations. that should get you real close.
you can also do the math with the tire diameter, exact engine rpm and using a GPS exact speed. but this introduces error in speedometer, tachometer, and tire size that you may measure. take several readings and average it out and you should get real close.
the are also several calculators in the internet that will get you real close.
here is an example of a 12:37 ratio stamped on the pinion gear with the date.. 12/37 = 3.08 ratio.
Last edited by joewill; Sep 13, 2010 at 01:19 PM.
Burning Brakes
Joined: May 2008
Posts: 860
Likes: 0
From: Stratford Ontario
You can also get a very close idea or confirm any of the above methods above by raising back wheels off the ground and car in neutral. Mark drive shaft with chalk then turn tire exactly one turn and count driveshaft rotation. I have 3.90's in mine so it turns just shy of 4 turns, 4.10's would turn just over 4 turns. 3.73's would be 3 and 3/4 turns etc...
