[veet66]

On my 65 GTO I am trying to figure out what size rearend I have. I do not want to open it up. It is a non posi rear. I turned the wheel one full revolution and the drive shaft turned 1 1/2 times. Can I determine the size from doing it this way?

Thanks,

[Dick Boneske]

It appears you're looking for the rearend gear ratio, not size. Your '65 most likely has a 3.23:1 gear ratio. The drive shaft would turn 3.23 times for every 1 turn of the rear wheel.

It sounds like what you've done is turn only one wheel one turn. If the other wheel was not turning, the drive shaft would turn 1/2 as many turns as if both wheels were turning. So, if your drive shaft turned 1 1/2 turns with one turn of one wheel, the ratio is about 3:1. To get a more accurate reading, turn the wheel two turns and see exactly how much the driveshaft turns. As I said, it will likely be about 3 1/4 turns (3.23 to be exact).

[b-man]

The easiest way to determine the ratio on an open (non-posi) rear is to leave one wheel on the ground and count how many turns of the driveshaft it takes to make the other wheel that's off the ground go around 2 full revolutions.

On a posi rear that's working correctly, get both wheels off the ground and count how many turns of the driveshaft it takes to make the wheels go around one revolution.

Mark the driveshaft yoke at the rear end and the tire for the best accurracy.

[richard sargent]

WHOO! IF IT WAS MY REAREND I WOULDN'T WANT TO OPEN IT EITHER! LOL!

[richard sargent]

This guy cut me off the other day in traffic I know what size his rearend was.......... BIG!

[chrisp]

Turn the drive-shaft at the yoke , not the wheel .

[veet66]

Thanks all, I did the test and found it is a 3.08. Looking at the book a 308 in a 65 was an option. The reason I wanted to know is I have an original posi unit I want to install in the car but before I open the rear I wanted to make sure the gears are what I want. I heard that they made 3 different posi's for different gears. Now that I know what I have does anyone know how to tell on the posi if it will work on those gears? The person I bought it from did not know what gears were on the posi. Is there a way I can measure the bolt holes on the posi to tell me if a 308 will fit?

Thanks again for all of your responses.

[Squidward]

Originally Posted by b-man

The easiest way to determine the ratio on an open (non-posi) rear is to leave one wheel on the ground and count how many turns of the driveshaft it takes to make the other wheel that's off the ground go around 2 full revolutions.

On a posi rear that's working correctly, get both wheels off the ground and count how many turns of the driveshaft it takes to make the wheels go around one revolution.

Mark the driveshaft yoke at the rear end and the tire for the best accurracy.

Hmmm... I tried this with my XF coded rear in my 67 bird (says it's supposed to be a 3.23 open, but it's not original to car). I had both wheels off the ground and I got 3 1/4 turns of the driveshaft for 2 full turns of the tire.

Is this correct, or was I all hepped up on goofballs? Does it matter that both wheels were up? Sorry about the basic question on an old topic.

Thanks!

[b-man]

On the open rear you need to leave one wheel on the ground, or make sure one doesn't spin. If you don't keep one side from spinning you will likely get an inaccurate count of the other one that you're trying to count 2 turns on.

The differential spider gears have a 2:1 ratio so that's the reason for the 2 turns of one tire with the other held in place.

[Squidward]

OK, thanks. I'll try it again with one wheel down. I did notice that the driver side wheel didn't move at all while I was rotating the driveshaft and the passenger side was spinning.

I'll repeat in the morning when I get home.

[b-man]

If the driver's wheel didn't move at all then you probably have a good indication of your gear ratio, looks like 3.23s.

Couldn't hurt to recheck with the wheel on the ground.