I am trying to figure out the rear gear in my 72 Cutlass S. I have used the driveshaft and tire rotation twice, coming up with 2.73 ratio. However when I compare to rpm, mph, and tire diameter, I come up with 2.93 ratio. Not to mention the diff cover looks like the GM 8.2 rear housing. I verified mph using a GPS and my tires are 245/45/18.

RPM 2400
MPH 65
Diameter 26.7

4A428194-B159-4B61-BF16-72027D7DFDA9_zpse7mbgzpz.jpg

 

counting driveshaft turns is hard to go wrong with. Mark the tire[s] for accurate turn counting, and the difference between "just under 2&3/4 turns" and "way past 2&3/4, damn near but not quite 3 turns" should be obvious by the obvious 1/4 turns per the U-Joint.

I have never understood why folks don't just pull the cover- on your own car or a diff you are looking to buy anyhow- and read the numbers stamped into the ring gear.

 

I put masking tape on the tires, so I feel the tire turns where accurate. I didn't plan on changing the gear oil and making a mess, but I guess I should just pull the cover.

 

If this is a non-posi rear, both wheels need to turn exactly the same amount, or you may get an incorrect reading. Alternately, you can block one wheel to guarantee that only the other is turning, then multiply the resulting answer by 2 (due to the 2:1 ratio in the differential). Also, since it can be difficult to tell the difference between 2.7 and 2.9 turns of the driveshaft, I always go ten turns of the tire and divide the result by 10. It's a whole lot more accurate to compare 27 turns vs. 29 turns.

 

If it's a stock rear it's likely the 2.73 gear set. That was the standard for 72 Cutlass S. Axle code SA. Cover is correct for 72 10 bolt.

Edit your tach could be wrong. The driveshaft and gear rotations never are.

 

Don't forget about torque converter slip.

 

That's part of the problem. The other part is that the tire O.D. you calculated is the theoretical O.D. without any weight on the tire. As soon as you load the tire, the sidewalls flex (more on radials than on bias ply tires), which reduces the effective rolling radius. A smaller rolling radius means the tire turns more revolutions per mile than the calculated number, so the engine RPMs will be higher than expected. I'm also guessing you have a 2.73 gear set.

 

Thanks for the responses. Everything points to it being 2.73 rear. I tried finding the stamp but didn't have much luck. A lot of good information.

 

The SA stamping is on the right side, same as the vent tube. It will have a stamping that indicates SA (2.73 code) O (olds) and the ordinal date it was produced. Mine happens to be SAO 0101 which happens to match the production date of my car to the day.

 

Here's a picture of the stamped gear ratio and date code on my original 2.56 10 bolt:

197110boltR2256opendiffcomp.jpg

 

Thanks for the details and picture. I need to do quite a bit of cleaning to find the stamp. At least I know where exactly to look. I have an assembly manual that I referenced, but a picture is worth a thousand words.