I have a 1966 cutlass supreme with the original rear. I recently replaced the st300 with a th350 that I rebuilt. I want to do the rear next, but my question is merely out of curiosity. I've pulled the cover plate on the rear last year to change the lube and gasket but forgot to count the teeth for the ratio. I tried turning the driveshaft and counting the tire revolutions and I come up with a 1.55:1 ratio?? Is this a realistic ratio (especially with the original st300) or do I have to resort to pulling the cover? The only codes I could find were an upside down SC on the back passenger side axle tube and 1375689 or I375689 on passenger side center section right below the webbing.
Any help is appreciated
Thank you
Dan

 

SC is a 3:08 gear set, 1:55 is not any ratio that you could get, 2:56's would about as low (numerically) that were available. Pulling the cover is easy, and to be sure after 47 years, that it hasn't been changed. Better to know than to guess.

 

Thanks 1969
I just redid it. I turned the wheel 20 times and what seems to be 30 and almost 31 turns (call it .9 or.8) on the driveshaft. So looks like its a 3.08 to 1. Now for the next question. What's a good gear ratio for this? 3.7x or deeper? Would I notice a difference from 3.08 to 3.73 ? I just got this car on the road, I thought it would have a little more get up and go. oh well.

 

That's the count you would get for ONE revolution of ONE tire. You need two revolutions of one tire, or one revolution of BOTH tires, or any combination in between that adds up to 2 spins. Twice 1.55 [no colon here, it's a decimal number] is 3.10 or right about 3.08. Voila'.


I just redid it. I turned the wheel 20 times and what seems to be 30 and almost 31 turns (call it .9 or.8) on the driveshaft. So looks like its a 3.08 to 1. Now for the next question. What's a good gear ratio for this? 3.7x or deeper? Would I notice a difference from 3.08 to 3.73 ? I just got this car on the road, I thought it would have a little more get up and go. oh well.
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There are not many useful ratios stronger than 3.73- 3.9, 4.1, very few cars can use 4.3 or higher. You lose highway driving ability.

They say 3.42 is the best balance of sport and economy. You might tolerate 3.73 if you run an OD trans- the '85 442 doesn't mind it. 3.23 is good, 3.55 exists...

 

What speeds will you be driving the most? 3.42:1 will put you at about 3200 rpm @ 70 mph. 3.23:1 will drop you to about 2700-3000 rpm. If you do mostly local driving/cruising 3.42:1 is good, also have to consider your carrier and what ring and pinion will fit.

 

If it is a 66,then it should have an 8.2" 10-bolt. I know there is a 3:36 and a 3:55,that would both work nicely. The 3:36 would not be as noticeable when changing from the 3:08 that is already in there.

 

I've spent the thinking about the direction of this build. It drives so nice with the th350 and the 3.08. Out of the hole I wish there was more, but its a four door so I may just leave the gearing and thghten up the suspension and call it a day. The other direction I want to go is junior stock drag car from the 60's, like a mid 13 second car with cheater slicks and a tad higher in the nose. Not sure what to do yet.