Help figuring 9 inch rear gear ratios?

unibody madness · Post by **unibody madness** » November 20, 2010, 2:05 pm

I just spent two hours researching this site, and one or two posts at fordification, on how to figure what rear gears were in the turk.
I know opening it up would be definitive, but do not have the time to waste on a rear I may not use( I have a rebuilt 3.50:1, I can swap in)

Turning the drive shaft with one wheel on the ground 10 times, my axle turned 5 and 1/8 times (twenty was ten and a quarter)

Turning the rear axle 10 times, my drive shaft yoke turned 19 5/8 times
(twenty was thirty eight and seven eighths)

Anyone help with a ratio?

charliemccraney · Post by **charliemccraney** » November 20, 2010, 2:54 pm

Something isn't right. Your ratio comes out to about 1.96:1 with those numbers.

I think maybe you counted 1/2 of the axle rotation as one. If it is treated like that, then your ratio comes out at about 3.90 - 3.92. The closest gear set is 3.89.

unibody madness · Post by **unibody madness** » November 20, 2010, 3:03 pm

Charlie,
I placed a piece of tape on the axle and bearing retainer,, did the same on the driveshaft yoke to housing then cut the tape on both before rotating
If you count ten rotations from yoke to axle, doesn,t it make the ratio 5.14:1?

charliemccraney · Post by **charliemccraney** » November 20, 2010, 3:34 pm

Unless I'm doing the calculations wrong, you definitely don't have a 5.14.

Using a 3.70 ratio as an example, the drive shaft will turn 37 time for every 10 rotations of the axle. It is the number of drive shaft turns / the number of axle turns, we'll say ds / axle. 37 / 10 = 3.70
In the first scenario, the drive shaft turned 10 times and the axle turned 5.125 times. ds / axle = 10 / 5.125 = 1.95
In the second scenario, the drive shaft turned 19.625 times and the axle turned 10 times. ds / axle = 19.625 / 10 = 1.96

If you counted 1/2 a turn of the axle as one, it will cut your axle revolutions in half, 10 / 2.56 = 3.91 in the first scenario, and 19.625 / 5 = 3.93 in the second scenario. The closest being 3.89. Counting 2 revolutions of the drive shaft as 1 will result in the same thing.

62galxe · Post by **62galxe** » November 20, 2010, 3:49 pm

Here you go. You need to turn the wheel twice as much with a open / non posi rearend.
What gear ratio do I have?
Jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posis and lockers and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading.

banjopicker66 · Post by **banjopicker66** » November 20, 2010, 4:22 pm

Are you sure you have a Ford 9" and not a Dana 44 with a locking rear?
Ford used Dana axles if the build order specified a locking rear end - not a 9".
The locking rear would explain the mis-count, as just described.

62galxe · Post by **62galxe** » November 20, 2010, 4:28 pm

unibody madness wrote: (twenty was thirty eight and seven eighths)

Anyone help with a ratio?

Sounds like a 3.89 gear.

unibody madness · Post by **unibody madness** » November 20, 2010, 4:49 pm

OK so as I first posted I counted both ways.
turning the drive shaft/counting the axle:
drive shaft got 10 turns= axle 5 1/8
......................20..................10 1/4
turning the axle/counting the drive shaft:
axle got 10 turns= driveshaft got 19 5/8
.............20.................................38 7/8

so according to 62 galaxy given a two to one spin ratio that makes it pretty close to 3.89?

If I could type faster I would know the answer to that!!!!!!!!!!!!!!!

Its amazing how you and charlie came up with about the same answer working from opposite ends
Thanks

Michelle · Post by **Michelle** » November 20, 2010, 6:45 pm

I am coming up with a lots taller ratio than that. I do not think it is that low. Turn the axle one full turn with one wheel on the ground and tell me how many times the yoke turns over. If it is an open rear end you can multiply the number of turns of the yoke by 2 and that will give you the correct ratio. In other words with one wheel on the ground you turn the wheel one full turn and you yoke turnes 1.5 times you multiply that by two and you have 3. That means you have a 3.00:1. From the figures you gave I think you have a 2.50:1

GDG · Post by **GDG** » November 20, 2010, 7:27 pm

Do it like 62galaxy said, it's always worked for me.
Doug

ProStreet66 · Post by **ProStreet66** » November 20, 2010, 9:53 pm

Mine had 3.89. I have a 64 with 223 6cyl and three speed. There should be a little tag on the rear that tell you gear ratio. Is on 1 of the bolts.

frdnut · Post by **frdnut** » November 20, 2010, 10:20 pm

Michelle wrote:I am coming up with a lots taller ratio than that. I do not think it is that low. Turn the axle one full turn with one wheel on the ground and tell me how many times the yoke turns over. If it is an open rear end you can multiply the number of turns of the yoke by 2 and that will give you the correct ratio. In other words with one wheel on the ground you turn the wheel one full turn and you yoke turnes 1.5 times you multiply that by two and you have 3. That means you have a 3.00:1. From the figures you gave I think you have a 2.50:1

Yes..This is the correct way to do it with an open rear end...
If you could get both wheels to turn equally you wouldn't need to double the yoke turns but it is very difficult to get both wheels to turn equally with the open rear..Better to leave one on the ground and double the number that the driveshaft turns...

robert porterfield · November 20, 2010, 11:43 pm

could just pul pinion out the front of unit......then mark and count the teeth,....then you'd know for sure

unibody madness · Post by **unibody madness** » November 21, 2010, 2:00 pm

Opening them up is not an option right now. as I am swaping rears between the 460, and the turk ,with a third rear member involved.
Kind of like a shell game with the rebuilt 3.50 rear ending up in the frame to the turk.

Michelle
I tried your method and came up with just under two complete rotations of the pinion gear which gets me in the ball park of under 4. something, but not an exact number. However I came up with 3.89 on both my loose rear members with 62 galaxies' method which is the same as yours but times 10, I will let you know later today if the 3.50 member math works out with the same method.
Thanks for all your help
John

Michelle · Post by **Michelle** » November 21, 2010, 2:04 pm

With just under two turns, you probably have something in the neighborhood of 3:70 or 3:89.

unibody madness · Post by **unibody madness** » November 21, 2010, 5:01 pm

Just checked the 3.50: rear end after it came out of the 460, with the ten/twenty turn method, and came up with 35 so the math works
Thanks to all for your help
john

Help figuring 9 inch rear gear ratios?

Help figuring 9 inch rear gear ratios?

Re: Help figuring 9 inch rear gear ratios?