70 BARRACUDA 1.34:1 GEAR RATIO **HIGH RPM @ HWY SPEED**

[70 Grand Coupe]

With your advice I figured out my gear ratio to be 1.34:1., 8 3/4 non sure grip. Sassygreen recommends 2.73:1 and Chryco Psycho suggests a 3.23:1 ratio. Just to confirm the higher the ratio, I will gain in cruising speed, but will loose the get up and go off the line correct?

Just a friendly reminder the car has a moderately modified 318 with 340/360 cam, Edelbrock Performance RPM Heads, RPM Air Gap intake and Thunder 650 cfm carb (new). The 904 TF was rebuilt before I bought the car so I would like to continue using it for now. I am quite impressed with the power the SB is making. Of course, I dream of stroking out the 440 I have on my engine stand, but that will come later.

In the interim, I would like to know the most cost effective way to gain a little cruising speed on the Hwy cuz right now I am at approx 60MPH at 3000rpm.

Q1: is more cost effective to change out gears or transmission as suggested prior?
Q2: If I was to change gears, do I have to change out axles? Any other hidden parts, costs?
Q3: Is it easy or difficult to convert to a sure grip while I am at it?

Thanks.

Darcy

 

[roadman5312]

       I think you need to recheck that. Never heard of a ratio that low.   

[CUDA JAS]

I agree...that does not seem right.

How did you determine the rear end ratio?

If you are turning 3000 rpm at 60 mph, my guess is you have closer to a 3.73 gear.

Jason

[70chall440]

From the sounds of it, you are probably running 3.55 or more likely 3.91 (common gears for a 8.75).

To your questions

1. much more cost effective to change the rear end gears. They are in a 3rd member aka "pumpkin" which can be sourced as a complete unit, making changing them very easy.
2. No, so long as the 3rd member you obtain accepts the splines of your axle (which they should). Only other think you need is a gasket (can use RTV as well) and gear lub.
3. 2 options here
   A. Purchase a sure grip pack and rebuild your 3rd member with new gears and the SG pack
   B. Purchase a complete assembled 3rd member that already has the SG pack (best option, lots out there)

As for gears, If you don't like the RPM on the hwy, then go with 3.23 or perhaps even 2.76 but you are going to lose the low end take off. It will feel slower at take off. I personally like 3.55 as they are good middle ground gear ratio.

[70 Grand Coupe]

Thank you gentleman for the quick replies and feedback. I think I have miscalculated my gear ratio. I lifted my rear end up off the ground, with the car in neutral I marked my tire and driveshaft. I turned the wheel (1) full rotation and counted 1.34 rotations on the drive shaft. I am obviously doing something worn because according to the Summit calculator my car with that gear ratio: 1.34, Tire diameter: 27" (255/60/R15) at 3000rpm = 180 MPH.......lol! However, I would need a tailwind, pointing downhill and 8 miles to achieve that  .

Something tells me to lift the car again, using the same marks, turn the driveshaft and not the tire to ensure proper rotation.......thoughts?

UPDATE: I just worked backwards on the SUMMIT CALCULATOR to figure out gear ratio with Tire diameter: 27", RPM: 3000. Speed: 60 and it gave me a 3.91. I look forward to your replies. Thank you in advance.


Darcy

[roadman5312]

              Yep, is you have an open rear end block one wheel from turning.       

[Chryco Psycho]

if only one wheel was turning you need to do 2 full rotations to get the gear ratio , if you double what you have it would work out around 2.76 ratio

[70 Grand Coupe]

Again, thanks for feedback. I gave one of the rear tires (2) full turns and the driveshaft turned 3.23 turns. But I am still somewhat perplexed  :bang head:. According to the Summit Gear Ratio Calculator, the 3.23 should give me 2500 RPM @ 60mph with 27" tires with a 1:1 trans w/o OD. As y'all know, I am at 3000 RPM with all the above.

Thoughts?


Darcy

[70chall440]

factory tach?

Sure it is shifting into 3rd gear?

[70 Grand Coupe]

After market tach, and I think your onto something. I think she aint shifting into 3rd. Would the kick down have anything to do with it not shifting into 3rd?

Darcy

[70chall440]

Yes, it must be adjusted corrected so as to allow the trans to shift. I am running a cable system on my 52 PU and it took me quite awhile to get it adjusted. Not hard, just takes some time. You have to make sure the kick down is adjusted as well as the floor (or column) shifter to ensure they are doing what they are supposed to be doing.

[734406pk]

You can also try upshifting/downshifting manually and count the shift points. Sometimes this works to define them. Good luck!