Hi,
I have no shame in admitting I know next to nothing about appropriate gear ratios but wanted to ask and get some advice. My 70' CS is a slightly modified 350 4 BBL with a TH350 automatic trans and stock rear end. I haven't found the stamp code on the axle yet (gonna wire brush it soon) but I believe the 70' CS was either 2.56 or 2.78 stock.

As my driving style is fairly conservative with no drag racing and mainly all weekend, city and some highway driving, I haven't felt the need to change the gear ratio to a different setup. What are the pros/cons and for a driver and not a racer, is the stock setup a good match for the car?

On a slightly related note, I don't have a tach hooked up yet to know what my RPM's are at cruising speeds but I assume they're in the normal range. If my understanding is correct, the RPM should be around the 2500-2800 range @ 65-70 mph cruising speed. Am I correct or in the ballpark? I'm confident the trans is shifting into all 3 gears smoothly, so when I do hook up a tach, if the range is higher than that amount, does that indicate an issue with the stock ratio and perhaps not ideal for my setup?

 

The rear tire size will affect the overall effective gear ratio, so a shorter tire will result in higher RPM and a taller tire will result in lower RPM with the same rear gears.

With a 2.56 rear gear and 27" tall rear tire, the engine should be turning around 2400 RPM at 70 MPH.

 

I am around 2350 rpm at 60 mph, 2600 at 70 mph with a 225/70R14 tire and a 2.78 gear.

 

If you're conservative, IMO I'd leave it as is and enjoy it. Changing to different gears has some advantages in performance but also requires changes to the drive/driven gears in your transmission to keep the speedo and odometer properly calibrated. That's not hard to do, just so long as you get the proper gear sets. If you want a bit more oomph in your city driving, you might consider 3.08:1 gears, which are a great compromise for city/highway. The standard gear for the 70 Cutlass Supreme was the 2.56:1 so that's probably what you have. They are a bit slow off the line but it's intended as a highway cruiser. Other available gears for 70 CS were 2.78, 3.08, 3.23, 3.42 and 3.91

 

If you bump your gearing, 3.08 or especially 3.23 would feel like a world of difference for acceleration & driving “in town” without stressing you out on the highway. You WILL feel like the trans didn’t go into high gear for the first few times but it will seem normal soon enough. While you’re at it, you might want to add limited slip differential to keep traction with your new found gear multiplication.

Gears, extras & install likely $1200ish, add $600ish for limited slip addition.

Back in ‘09 when kid’s red car hit the road we did exactly what you are thinking about, installed 3.08 & limited slip. He was slightly traumatized the first couple of times he drove it on the highway but liked it for local driving. Then in went a nice used BRF 200-4R overdrive transmission and pure bliss. I think car tachs around 1800 rpm locked up at 70 mph. This stuff can be a never ending adventure!

 

Well if all you want is a bunch more pop or you want to spin the tires just bump up the stall converter from a stock 1350 -1500. Go 2000 -2500 stall and the tires will snap loose pretty hard and quick.
If you are into a bit more acceleration the gears will help but remember you have to get traction and that gets harder with street tires - you will feel a seat of the pants feeling more with more gear for sure.

Have you done any work to the motor? Remember doing too much camshaft duration will kill your low end torque and seat of the pants feeling.

Hope more info help you make the correct decision for your car.
Jim
JD