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Identifying Rear End Gears
#1
What is the easiest method for identifying the ratio of the gears in my rear end? My dad is almost positive the original owner put in after market gears and we were wondering what they were. I know from the vin decoder that it came from the factory with 3.50:1 gears.
'73 "Q-Code" Mach 1
[Image: Mt7Cz.png]
|351C-4V|Holley 600CFM|Edelbrock Manifold|Unknown Cam|10:1 Pistons|Hooker Longtubes|Glasspacks|C4 Auto|Unknown Gears|

Cold Start And Run Video: http://www.youtube.com/watch?v=Kqr4NVCHxBk
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#2
(11-22-2010, 01:41 AM)bkelley Wrote: What is the easiest method for identifying the ratio of the gears in my rear end? My dad is almost positive the original owner put in after market gears and we were wondering what they were. I know from the vin decoder that it came from the factory with 3.50:1 gears.

This is from a previous thread written by Cobra:


cobra3073
VMemphis, TN
Reputation: 10


As to rear end ratio, there are simple means to determine the ratio both with the pumpkin "in" and "out" of the car.

1. If the rear end is still in the car, you will need to jack up the rear, maybe by using jack stands. Mark one of the rear tires with a piece of chalk at the 12 o'clock position. You will also need to put a horizontal line on the drive shaft. Rotate the tire one full revolution while watching how many times the drive shaft goes around. If it goes around 3 1/2 times (and you have a posi in the rear), that would denote a 3.50:1 gear. If it goes around 3 3/4 times (again with a posi), you probably have something like a 3.73:1 gear.

If you do not have a posi, all you have to do is multiply the revolutions by 2. For example, if the driveshaft rotates 1 1/2 times when you you make one revolution with the tire, you multiply that by 2 which will yield a 3.00:1 open rear gear.

2. If the rear end is out of the car, you simply have to count the teeth on the ring gear and the pinion gear and divide the pinion gear number into the ring gear number.

For example, if your ring gear has 37 teeth and your pinion gear has 9 teeth, the result would be a 4.11:1 gear ratio.

If the gears are still in the housing, you can usually count the respective teeth by placing a horizontal chalk mark on the ring gear and rotating the gear to determine the number. You would do the same thing on the pinion gear. Then do the division math as cited above.

Hope this helps!

BT

--------------------------------------------------------------------------------
Do the RIGHT thing.
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#3
Cool, I figured it was something like that. Thanks.
'73 "Q-Code" Mach 1
[Image: Mt7Cz.png]
|351C-4V|Holley 600CFM|Edelbrock Manifold|Unknown Cam|10:1 Pistons|Hooker Longtubes|Glasspacks|C4 Auto|Unknown Gears|

Cold Start And Run Video: http://www.youtube.com/watch?v=Kqr4NVCHxBk
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#4
the only problem with the drive shaft rotation method is you need to be very accurate with your measurements of travel.

what happens is when your ratios are close to other ratios.

for example you have ratios like
2.75
2.8
2.86
2.91
2.94
3
3.07
3.09
3.25
3.33
3.4
3.45
3.5
3.55

it might be easier to visually see the difference between 2.75 and 3.25, or 3.25 and 3.5 or even 4.11

but tring to tell the difference between 2.91,2.94,3, 3.07, 3.09 will be very hard.

i will tell you from experience, i could not see enough difference between what i thought was a 3.00 ratio and what turned out to be a 2.75 ratio.

the 100% sure way is to count gear teeth.

another way to fall into a ball park is the Transmission speedometer gear, you can count the teeth on that and check that you fall into a gear range.

lets say you think you have a 3.25:1 ratio you pull the speedo gear from your C6 and count 15 teeth and you know your speedo is pretty acurate.
then there is no way you have a 3.25 ratio because you would not fall into the range of error of the speedometer. you would need a 18 tooth gear
to be in the 3.25 range, a 15 tooth would put you in the 2.75-2.8 gear range. 16 teeth would put you into the 2.9-3.1 range. and so on.
each will have a degree of error depending on your ratio being in the "zone"

when you start getting into 4.00 range now your talking about needing an external gearbox for the speedometer gear to get it to turn that fast.
thats why the SCJ cars with low ratios had that little external speedomoter gear adapter installed.



so as an example you think you have a 3.25 ratio you pull the transmission speedo gear and discover a 19 tooth gear that puts you in the range
of a 3.5 not a 3.25 so more likely you have a deeper ratio then you think.

example 2 you think you have a 3.25 you pull the speedo gear and find a 16 tooth gear that puts you in the 3.00 range not 3.25
so more likely you have a taller ratio then you think.

its really hard to be accurate under the car and measure things 100% a visual check and count on the gear teeth will be 100% correct.
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#5
Can you count the teeth with everything still in the pumpkin?
'73 "Q-Code" Mach 1
[Image: Mt7Cz.png]
|351C-4V|Holley 600CFM|Edelbrock Manifold|Unknown Cam|10:1 Pistons|Hooker Longtubes|Glasspacks|C4 Auto|Unknown Gears|

Cold Start And Run Video: http://www.youtube.com/watch?v=Kqr4NVCHxBk
  Reply
#6
If the "pumpkin" is out you can mark one of the teeth on the ring gear with colored chaulk. You can then rotate the pinion gear by hand while counting the teeth on the ring gear.

After doing the ring gear, you can do the same thing with the pinion - - place a colored chaulk mark on the pinion gear and rotate the ring gear while counting the teeth on the pinion gear.

Dividing the number of teeth on the pinion gear into the number of teeth on the ring gear will give you the ratio.

Hope this helps.


BT
Do the RIGHT thing.
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#7
just a thought but to distinguish if its a possi or open carrier, gear counting will generally not help..... you can spin the drivers side rear tire and if the passenger side spins in the same direction you have a possi unit, if you spin the drivers side and the passenger side goes the opposite direction then you have a open carrier. this also works spinning the tire either way forward or reverse.this is a quick tech tip with out tearing the rearend apart to find out...of coarse this is with the car on jack stands.......gear ratios can be hard to figure out spinning the tire it is best to take the tire off and use the axle or brake drum to count the revolutions,remember differant tire sizes give differant ratios , its close, but to be sure of the gear i recommend counting teeth on ring gear and pinion gear.
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