Possible to calculate gear ratio with given parameters?

[Michael_]

Car: 1969 Dodge Charger RT
Engine: 440
Transmission: 727
Wheels & Tires: 14x5,5" Magnums with 205/70r14
RPM & Speed: ~ 3000rpm @ 60mph
Everything stock

Given these parameters, is it possible to calculate the rear gear ratio?
Buildsheet is most likely somewhere in the car but i dont want to take the interior or rear end apart right now.

But it would be good to know what rear gear ratio my car has (roughly).

 

[Cranky]

You could also roll the car one tire Revolution and count how many times the drive shaft rotates.....but there are also online calculators that would get you real close with the info you provided.

 

[Beekeeper]

You could also roll the car one tire Revolution and count how many times the drive shaft rotates.....but there are also online calculators that would get you real close with the info you provided.

this ^^^ it’s the easiest way to do it. Jack the rear tires off the ground and mark the tire and wheel opening with tape and turn the tire one revolution and count the drive shaft revolutions.

 

[33 IMP]

Calculates to 3.58. So....3.55 if an 8 3/4, 3.54 if a dana.

 

[Pikser]

I’m impressed !

 

[Michael_]

Calculates to 3.58. So....3.55 if an 8 3/4, 3.54 if a dana.

Yes its a 8 3/4".
Thank you guys very much!

 

[33 IMP]

For future reference, check out Wallace Racing calculators on the net. Lots and LOTS of terrific automotive calculators. That's where I went, plugged in tire diameter, your rpm at mph, educated guess at converter slip and out pops the answer.
I use Wallace several times a week, usually.
Edit I used 25.3 for tire height, per google for your tire size,and 5% converter slip. If you plug in 6% you get dead-on 3.55.

 

[Don Frelier]

That's a pretty small tire.

 

[Michael_]

That's a pretty small tire.

I agree.
At some point i will buy a second set of bigger wheels and tires for sure.

 

[Dipstick]

this ^^^ it’s the easiest way to do it. Jack the rear tires off the ground and mark the tire and wheel opening with tape and turn the tire one revolution and count the drive shaft revolutions.

That only works with Sure-Grip, with an open differential, raise only one tire and turn the tire TWO turns.

 

[5.7 hemi]

Huh?

 

[33 IMP]

I agree.
At some point i will buy a second set of bigger wheels and tires for sure.

Well, if you go to a 28"tall tire, your rpm at 60mph will drop to a hair over 2700, your 3000 rpm will get you 66 1/2 mph instead of 60.

 

[1 Wild R/T]

That only works with Sure-Grip, with an open differential, raise only one tire and turn the tire TWO turns.

Huh?

With an open differential when you lift both wheels they are are free to spin with no effect on the driveshaft... The spider/side gears transfer the rotation from one tire to the other tire....

If you lock one tire (leave it on the ground) now the drive shaft is directly coupled to the tire that is free to rotate however the spider gears still effect the connection with a 50% reduction of the motion... So two rotations of the free tire are counted as one....

 

[5.7 hemi]

But you forgot to count the yoke spinning, so the trans has to be in neutral? And if the trans is in neutral, then only 1 tire in an open diff has to be spun and only 1 time?

 

[1 Wild R/T]

But you forgot to count the yoke spinning, so the trans has to be in neutral? And if the trans is in neutral, then only 1 tire in an open diff has to be spun and only 1 time?

Go out & give it a try... Let us know what you discover...

 

[5.7 hemi]

Just watched a couple YouTube vid, they are doing it with a “posi” and turned the yoke (after marking it) against the drum (marked also).

 

[1967coronet]

Ok, I have 245/60/14 on the rear of my 67.
I installed the 3:23 sure grip center chunk so I know what it is.
the 205/70/14 tire is advertised at 25.3 dia the 245/60/14 is 25.6 so they are pretty darn close.
My car rpms are 2,600 at 60 mph so that 3:55/3:54 ratio posted above has to be on it.