Hi all,
I'm redoing my leader board for the upcoming show season. What was the standard gear ratio range for a 10 bolt non posi rear axle ? I own a 1972 Cutlass Supreme conv. with its original drive train, But I don't have the original window sticker or broadcast sheet. The car is equipped with a 350 Rocket and 4BBL V8. and TH350 3 speed automatic transmission. Also equipped with A/C if that makes a difference. Any prompt help would be much appreciated. I need to get the board out for printing before an indoor show in mid March.

Any help would be much appreciated ! Thanks Tony

 

From what I recall seeing posted here on the forum, the base rear for the 1972 Supreme automatic trans may have been the 2.73:1 ratio. This is a bit different than both my 1970 and 1971 Supremes that had the 2.56:1 rear as the base no-cost ratio.

 

Thanks Fun71,
Yes the standard rear seems to be a 2.73:1 gearing. But then some one on Oldsmobile Club of America chimed in saying his 72 Cutlass Supreme with AC came with 3:08 gears, Clouding the issue a bit. Other replies had the TH350 with 2.73 gears, but no A/C !.

 

Rather than guessing, we could just look at the factory literature. 2.73 was standard, 3.08 and 3.23 were optional. This has nothing to do with A/C or not. The 3.23 was not offered in Calif.

 

Thanks Joe.
Tha car was actually an 8-71 build, but 72 model I went to the garage that my client is letting me use Sadly the car is on the ground and tough to get under... but I did LOL ! The entire axle has been undercoated and I could not get to the numbers,nothing visible. And I have a show in a month with no time to drop, strip and redo the whole axle. Most are saying 2.73:1 gears or 3.08. I scaned the whole axle for any signs but nothing for codes. My Dad had the car for 18 years in Maine and no maintenance records. So I'm not sure if it is even the original axle since we are 3rd owners of the car. But definately looks like the original axle. Only other thing I got was pictures of the diff cover if they varied between 2.73 and 3.08.

 

This is the 10 bolt diff cover. Only real thing i could photograph.

 

The diff cover and the entire rear are the same between the various gear ratios, so you need either the code on the axle tube or pull the cover and read the stamping on the edge of the ring gear.

Here's a pic of my 1971 rear showing the location of the code:

 

LOL you can sort of see what I'm up against. Whole axle is undercoated. I am 3rd owner. My Dad bought it in 1980 no window sticker or braodcast sheet. But the car is 8-71 build but 72 model Cutlass Supreme conv. Power windows and A/C. Whole axle is covered like this in the picture

 

Unless you can find the stamped code, there is no way to know by looking at the axle which gear ratio is in it, or if it even has the original gears after HALF A CENTURY. Your options are 1), find the stamped code, 2) pull the rear cover and look at the markings on the ring gear, or 3) use the "count the turns" method to determine the ratio.

Or, since no one reading your show sign will know if you are BSing them or not, pick a ratio and print it on the sign. Done.

 

Thanks Joe,
I might do the bored with a 3,08:1 ratio on the bored since the car is a mild modify anyways. Not sure if I'm doing the board right now anyways. Other things came up. But at least now I have time to research more. I'd scrape the axle for the code but I have an indoo national meet in a month and don't want tostart messing around with the axle. Plus don't have the facility to do the work in. But thanks for all the help. I had someone do a reproduction window sticker ( aged to perfection ) But he just did a line with TH350 automatic transmission, but did not include a code for the gear ratio of M38 or ( G90 )a.

 

Takes only several minutes to determine the rear-end gear ratio - several good YouTube videos. Here's one, there are many more. I do the one turn method, then I do ten turns and they should both match up with ten more revolutions equating to the same as one revolution only ten times more revolutions.

My own 71 CS differential numbers were:

LH Driver's Side Wheel Locked in a stationary position (Open Differential) yields:

x1 Wheel Rotation : x1.25 Yoke Rotation = 1:1.25
x10 Wheel Rotations: x12.75 Yoke Rotations = 10:12.75

Multiply Yoke Rotation x2 (Open Differential) yields:
x1 Wheel Rotation = 1:2.50
x10 Wheel Rotations = 10:25.5

Rear Differential Gear Ratio = 2:56 (41:16 Gear Teeth) - Open Differential

 

I always recommend using ten turns of the driveshaft. It's a lot easier to see the difference between 32.3 and 34.2 turns of the wheels instead of the difference between 3.23 and 3.42 turns.

 

^^X2^^