Determining gear ratio on a 741

[Fred69Runner]

Hi guys,
I'm trying to figure out what gear ratio I currently have in my 69 RR that had a 741 installed. According to the manual I should count the number of rotations of the pinion (driveshaft) with one rotation of the axle shaft (tire).
So, I jacked it up and marked the pinion at the u- joint and marked the tire and turned one rotation of the tire and counted one 1.5 turns of the pinion. It makes no sense to me, there is no 1.50 gear ratio that I'm aware of. What am I missing, did I did this wrong? Help please! Thanks

 

[Russ69Runner]

One tire still on the ground rite. If it has not been changed then it would be a 2 to 72 gear ratio. That is what mine had call it a granny gearing. Try turning the pinion and see what it take's to get the tire to turn a full turn. But sounds like you have the granny gear's. If you go into the rear end and replace the bearing's on the axel's then go with the green bearing kit. No need to adjust end play. Got mine form doctor diff and the new ring and pinion. Went with the 3.23 gearing. Now looking at what other people say might have gone with the 3.55 gearing. Less RPM at 60 MPH. I will have to see when driving the car.

 

[Fred69Runner]

Hi Russ, thanks for the reply. Yes tire on the ground, first left then right and got the same reading on both sides. As suggested I turned the pinion and it took 1.50 turns for one revolution of the tire. Here a pic of the differential.

 

[Fred69Runner]

With those gears it does 80 down the interstate and its not even working hard.

 

[69hemibeep]

With both wheels in the air you have to spin the wheel two times to figure the ratio. so the # of spins of the drive line to two rotations of a tire which = 1 for an open diff

 

[Fred69Runner]

Thank you SBSW, That gets it into the ball park. Think I have must have a 2.76 to 1. Thank you for the clarification!

 

[Russ69Runner]

Think that is what I came up with when checking mine that way. Yes it will leave the stop sign like it has an anchor tied to it's tail but on the open high way you would run at 80 with little RPM's.

 

[Fred69Runner]

Ya, but I can light up that right rear pretty good when I want to!!!!

 

[Russ Hood]

I have the 742. What would the popular gear ratio for this rearend? 3.91 possibly?