djclause

I know I have a 4.11 but I think it might be a 4.56...

at 65MPH, I am at 3500RPM
205.75.15 tires
M21 4 Speed
327/365hp motor...

is there a way to figure it out from there???
I want to go to a 3.36 posi rear but heard changing the gear is a pain..

Vette-66

Jack the back end off the ground, put a piece of tape on the drive shaft and mark the tire. Rotate the tire 1 revs and count number of revs of the drive shaft.

4.11 = 4.1 revs of driveshaft for 1 rev of tire.
4.56 = 4.5 revs of driveshaft for 1 rev of tire.

I think that gets you pretty close.

Chuck

JohnZ

Your numbers calculate to a 4.33 with 27" tires, so you have some speedo or tach error; Chuck's method above will tell you exactly what you have.

sjaroslo

Rather than trying to determine if that last rotation of the driveshaft was a ".1" or a ".5" rotation, try turning the tire 10 rotations. Should cut down on the speculation.

65nassau

I've been meaning to do this as well and asked the same question 6 months ago. (Gotta get to this stuff a little faster!)

Don't you have to hold the wheel opposite the one you are turning STILL if you don't have a posi rearend?

I think I heard that .....

JohnZ

If you don't have a Posi, have a helper turn the wheel on the other side the same full turn you apply on your side.

truble

Vette-66 has the correct answer. Guess you could also take the Ring and Pinion gear out and divide the number of teeth on the ring gear by the number of teeth on the Pinion gear, however Vette=66's answer is just as good and takes a whole lot less time.