what gears am i running???????????

[coldgin]

i have a 69 3/4ton gmc 4wd all original except it has a 700r4 and just completed a tbi conversion...what gears come in that truck and how can i tell if possible by a vin code maybe....i am thinking it is a 4.10....not sure...just lazy and havent looked at the rear end to see if i have a tag on it...

[67c10rustbuket]

On the 12 bolt axles there is a code stamped on the passenger side axle tube, on the front near the top closer to the diff. Someone on here can probably decode it for you. Or just do it the greasy way and open the rear cover and look at the gears, if they are original there will be teeth numbers stamped on the ring gear like 11:44 this would be 3.73:1 ratio where 44 divided by 11 = 3.727272. If there are no teeth number stamping you will just have to count the teeth and divide.

[CC69Rat]

Isnt there a way you can do it by turning the tire one time and counting the times the driveshaft spins ? Something like that? (on a jack of course)

[PNWchevrolet]

Yes, there is a way. I was able to identify my differential type and gear ratio with the rotational count method. Here are the steps:

Preparation:
1) Secure vehicle from movement by blocking front wheels
2) Lift the rear off the ground (any method as long as both tires spin freely)
3) Put the vehicle in the correct gear neutral
4) Put a chalk mark on the drive shaft
5) Put a chalk mark on one of the rear wheels

Determine type:
1) Spin a wheel in one direction and view the spin of the other wheel
2) If it spins in the same direction you have positraction
3) If it spins in the opposite direction you have an open differential

Preparation:
1) Block the opposite wheel so there is no spin
2) Note position of all chalk marks

Determine gear ratio:
1) Spin the rear wheel 10 rotations while counting the driveshaft rotations (you may need help if you cannot keep both counts both simultaneously.
2) Apply the following formula: Driveshaft rotations * 2 divided by 10

Here is the math which determined my ratio:

Wheel rotations: 10
Driveshaft rotations: 18.65
Multiplier: 2
Multiplied driveshaft rotations: 37.3
Multiplied driveshaft rotations divided by wheel rotations: 3.73

Hope that helps!

[coldgin]

thanks for the input...i am going to try the wheel rotation...i will let you know what i find out

[PNWchevrolet]

What did you find out?

[coldgin]

never checked on it...sorry...I am still curious though....I am assuming it is a 3.73...truck is all original from the factory except tbi retro fit and 700r4 conversion

[PNWchevrolet]

Thanks for checking back. It has been a while for me.