I just calculated RPM using one of those on line calculators.

With 3.73's and 215/75R/14's with a four speed in my 70 Cutlass Convertible I am at 3700 plus--at 80 MPH! Ugh!!

With 2.73's and same tires and four speed I am at 2700 plus at 80 MPH. Ok.

I cannot live with 3700 RPM. I could live with 2700.

Perhaps different tires is part of the equations too. Much taller. Thinking about swapping in 15 inch SSII's.

Any suggestions on tire size for my equation is appreciated too.

Qustion: With a 455 at 450 HP and 500 Ft Lbs of Torque, is this car going to be a dog with 2.73's?

It's a convertible cutlass highway cruiser. Montana freeways just went to 80 MPH. But I want a fun, fry the tires posi for the street, light to light. Car will never see a drag strip, but should feel like it could. Does that make sense?

Suggestions on an appropriate combination of tires, gears is appreciated. Not going use an overdrive.

Final questions: Will a 3 series used original GM O type posi work with my 2.73 non posi gears currently in the car?

Thanks.

 

I edited my post, you will be disappointed with 2.73 gears. Most discussions revolve around automatics and the suggestion is an OD trans. In your case, I'd look into a gear vendors and go with the 3.73, its only money...

 

215/75R/14's paired with a 455 with 450 HP and 500 Ft Lbs of Torque will certainly provide lots of tire smoke. Before swapping gears (and even afterwards) I would look into larger rear tires, such as 275-60-15s.
No, you need the 673 carrier for 2.56 and 2.78 ratios. Luckily, this is the least sought after O-Type limited slip carrier so it may be easier to find.

Below is info I archived from Brian Trick many moons ago:


Here is a list of the posi units & what gears they will fit:
#673 = 2.56 & 2.78 ratios, 28 spline
#671 = 3.08 & 3.23 ratios, 28 spline
#588 = 3.42 & anything numerically higher, 28 spline
#672 = 3.42 & anything numerically higher, 31 spline

 

1) Where are you getting 3.73 gears for a Type O?

2) 215/75-14 tires are already 26.7" tall. The 255/60-15s on my H/O are 27.0" tall and I don't find the 3.91 gears objectionable on the freeway. 3700 RPM is not particularly stressful for an Olds motor.

3) Any non-OD trans will have the same 1:1 final drive ratio. If you can't handle the RPMs, get an OD trans.

4) You REALLY don't want 3.73 gears with a manual trans, even with a BBO, unless you have a much steeper first gear. Your clutch will thank you. With the 2.56 first in the Muncie, your effective ratio in first is only 6.99:1. You want close to 10:1 effective ratio. Assuming you could get 3.73 gears, the effective first would be a nice 9.5:1.

5) The only other low-cost option is to source a Super T10 trans with the 2.88 first. With your 2.73 gears you get 7.86:1 effective in first. Not Pro Stock territory but at least you won't burn up the clutch.

 

How big a tire will fit in the 70 Convertible wheel wells? No scraping allowed. Clean fit in the wheel well.

Joe--pulled that 3.73 out of a hat. Probably going to be Richmond gears at 3.42's or 3.90's. With that M20 how do you think I am shaping up?

 

It depends upon the wheel backspacing, which needs to be matched to the tire size being used. I think Brian Trick has 325-50-15s under his car, but he said it is a close fit. 295-50-15s will fit with no issues.

 

Let me simplify this equation. I would like to run SSII 15 inch wheels on all four corners.

Question: What is the tallest and widest tire I can run with no interference? Don't want to roll the wheel well lip. Wide is not really my issue. Tall is. Trying to get the RPM's down.

Thank you.

 

The answer will depend upon the wheels. If you are using factory 15x7 wheels, 275-60-15 is about the largest you can fit.

edit: I just saw you want the same size all around. The widest are 245-60-15s and will fit with no issues; some have said their car worked with 255-60-15s but the fronts on mine rubbed with that size. It probably depends upon the ride height of the car and how stiff the front springs are. I think the tallest tire that is still made is 225-75-15.

 

If you're running a Type 'O', 3.42 is probably going to be as low (numerically high) as you can find. I've been pretty happy with my 3.08.

 

Do you have an automatic or 4-speed with the 3.08 gears? If a 4-speed, is it a close or wide ratio?