Question About Rear Diff. Gears for 8 3/4

[JTG]

Hi, I know 3.91's are not the best highway gear, just wondering what cruising speeds can one expect from 3.73 and 3.55, Thank You

 

[Burntorange70]

How tall is your tire?

 

[Burntorange70]

Here this way you can play around with it some.

All you realy need to know is the RPM, tire hight, and gears.

Then use this to find out.

Tire hight divided buy two divided buy gears X RPM divided buy 168.


So lets say

26 divided by 2 divided by 3.23 X 6000RPM divided by 168=143.74

 

[moper]

Yeah, you kind of need to give more info... What rpm do you want, what tire size, what engine/trans... Etc

 

[Supershafts]

What Trans ? what is your target rpm ? and what is the height of the tire ?

 

[JTG]

340, xheads, cam has a 520 lift, sorry duration doesn't come to mind at the moment, Edelbrock torquer2 intake, 750 Holley d.p with mech.sec., the tranny is the 727 that M.P once offered, stall converter is a 10" Turbo Action 3200-3500 range, Hooker headers, the gear in the back at the moment is a 3.91, the tire size is just a tiny little bit over 25 inch. This is a car that has not been driven yet. It will be more street then strip, just wondering if there's another gear one can use with this setup and still be able to cruise on the highway where one can at least go with the flow and not have RPM's going way through the roof. I know alot of people are going to say leave the 3.91 where it is and screw the highway, just want to know the different options I have, thanks.

 

[Burntorange70]

So much for teaching a man to fish. LOL Ok I will do the math for you.


With the set up you got now you will be doing about 57MPH at 3000RPM

So to go with the flow of 70MPH on the hwy you will be running 3750RPM


With a set of 3.55 gears you would be running 3500RPM and doing about 73MPH.

With a set of 3.23 gears you would be running 3000RPM to do 70MPH.

 

[JTG]

Thanks Burntorange, sorry never good at math.

 

[Supershafts]

But you will need to be at over 3000 rpms or you're going to have issues with the TC... TC slip brings up temps...not keeping it locked is bad...

Martin Saine has a new TC that will do whatever and then lock up completely like it was clutch.... much better for a street car, and race car since there is no loss in the TC...
Like having the old clutch flite.

 

[fishy68]

So much for teaching a man to fish. LOL Ok I will do the math for you.


With the set up you got now you will be doing about 57MPH at 3000RPM

So to go with the flow of 70MPH on the hwy you will be running 3750RPM


With a set of 3.55 gears you would be running 3500RPM and doing about 73MPH.

With a set of 3.23 gears you would be running 3000RPM to do 70MPH.

Don't forget to add a couple hundred rpm for torque converter slippage.

 

[Burntorange70]

Thanks Burntorange, sorry never good at math.

Sure thing. Hope you find a set up you like.

But you will need to be at over 3000 rpms or you're going to have issues with the TC... TC slip brings up temps...not keeping it locked is bad...

Martin Saine has a new TC that will do whatever and then lock up completely like it was clutch.... much better for a street car, and race car since there is no loss in the TC...
Like having the old clutch flite.

Don't forget to add a couple hundred rpm for torque converter slippage.

Good points. I am a 4 speed guy and don't know much about autos.