This includes many aspects of engine, transmission, rear differential and possibly instrumentation so I started here. Facts as I
know them. 1970 442 convertible, 455 CI,4 speed M21, Possibly born as an automatic but now a 4 speed. Posi diff. Tick Tock Tach, Wheels 14/7 I think 235s. Car in the shop right now so can’t check. New car so just getting acquainted with all the nuances. I would like to know if the following numbers “add up”. On the highway 4th gear. 3000 rpm = 45 mph 4500 rpm =70 mph. It seems to me this indicates at least a 3:91 ? Rear ratio or something even lower. Oh yes it has never seen a fuel stop it doesn’t like. This car is not numbers matching but has F casting heads and a w-30 intake manifold. Block is a 71 big car block.
what diff ratio do you think it is? Speeds seem to match traffic so I think they are close to reality.

 

If you are going by the factory tach, check it to make sure it is correct. So many factory tachs are not correct. My 70 4SP, with 3.91 gears and 15 inch wheels, runs about 3300-3400 at 7MPH.

 

Oh yeah that adds up to a lot of gear. With my probable 2.78 my engine is turning around 2000 RPM at 50 MPH and 3000 RPM is like 80 MPH.

Your car has been thoroughly handled / modded so really hard to pin point what gear it was/is. But can present maybes.

1970 442 Convertible auto came 3.08 standard gear but could be below or up to 5.00 optional.
1970 442 Convertible stick came 3.42 standard gear but also 4.33 4.66 5.00 optional.


Running your specs through a calculator comes up that you may have a 4.66

Keep in mind our speedos can be way off and so can a tic toc tac after all these years. My speedo is 10 MPH off at 60 MPH... but my tach is aftermarket new.... You could very well have a 3.91 but everything is so off that it shows like a 4.66

 

There are several online calculators you can use to determine your rear differential gearing. You need to know your transmission final drive ratio (usually 1:1, unless you have an overdrive gear), your tire diameter (how tall in inches), and exact rpms at a specific speed for input into the calculator. The below link will take you to one of the calculators:
https://www.differentials.com/techni...io-calculator/

 

Sounds like 4.33's to me. Could be 4.66,depending on tire size.

 

Gentlemen Thank You. I was initially thinking 4;11 as that is a number i often heard as a sprout. 4;33 and 4:66 had not entered my sphere but now has really piqued my curiosity.

 

The calculator at the link below is much simpler and more straightforward than the one at the link I previously provided. What's really nice about this one is that if you know 3 of the 4 variables it will solve for the 4th, no matter which variable you need to know. Hope this helps as you go forward.

https://www.strangeengineering.net/g...io-calculator/

 

I think my mystery was solved. With the car up on the lift my professional mechanic friend used an equation and came up with a suspected ratio of 4.55.
I will change this in the future to a lower ratio as the rear end is too low for comfortable highway speeds. Looking for a recommended Ratio. I’m thinking 3.73 or close to that. There are many choices and I’m soliciting opinions from those of you who are so helpful. I’m continuing my education in things Oldsmobile and particularly 442 Thank you.

 

Without knowing the tire size, it is impossible to provide useful information. "Wheels 14/7 I think 235s." is not sufficient information. 235/60-14? The aspect ratio matters. And by the way, if that's what you actually have, those tires are a Honda-sized 25" in diameter, which is nearly TWO full inches smaller than OEM tires for this car. This will boost RPMs at speed. Also, do you know your speedo and tach are both accurate?

 

It sounds as if this may be an O-Type rear, and if so, the available gear ratios are very limited, especially if you have 4.33 or 4.66 gears now. That would mean a 588 carrier and there are NO (as in zero) aftermarket gears made to fit that carrier, so you would need to search for factory 3.91 or 3.42 gears (those are the other ratios used on the 588 carrier). The only aftermarket gears I know of are made to fit the 671 carrier, which was originally used on 3.08-3.23 gears.

Richmond Gear advertises 3.42. 3.90, and 4.10 gears to fit the 671 carrier, and there is an aftermarket carrier available for the 3.08-3.23 series gears (it's a Ford 8.8 that is machined to fit the O-Type gears and bearings).

https://www.richmondgear.com/wp-cont...n1.pdf#page=32

Info on the O-Type:
https://www.supercarsunlimited.com/i...id/diff_id.htm

 

Joe and Fun 71. Thank you for your answers and comments. I so appreciate your assistance and knowledge. I’m off to the shop to visit the 442 and will specifically look at the tire size.

 

Bandit60,
When you get to the point when you're trying to decide the final rear end setup you want to go with, I can't stress enough, that in addition to knowing the diameter of the tires you intend to run, that it is also just as important to know that your speedometer and tachometer readings are accurate. If one or both are off, and especially if both are off in the same direction, you probably won't be happy with your results. I would suggest checking your RPM's with a timing gun with RPM functionality, and your speedometer with an inboard GPS unit. Only when you know all three of those variables are correct, will you be able to choose a rear end ratio that delivers the results you desire.

 

I would think if you have a 26 inch tall tire, 3.42 - 3.73 will make for a nice drop from a mid 4s gear but still be moderately aggressive.

 

Why not remove the cover and verify what carrier is present, what he gear ratio is and eliminate the guess work? You can confirm the limited slip, change the oil, remember to include LSD additive, maybe check the breakaway torque while you’re there. I would be like the bank robber in “Dirty Harry.”

”I gots to know!”

MEASURE the tire diameter/circumference and then you can play with online calculators to see what you’re actually spinning and whether your tach is in the ballpark.