how to tell gear ratio

[HotRodPC]

And if you do pull the cover, the ring gear will be marked with the tooth count on both Ring and Pinion, so you don't actually have to count the teeth.

 

[78c20]

Gearing

You are pretty much on target C4 with one exception. You are 100% correct on a locker where you'd also have to jack up both wheels and the clue here is IF BOTH WHEELS TURN THE SAME DIRECTION. So if both wheels turn the same direction, then you are right on the money, except that I like to say if its 3 3/4 turns which in a fraction is 3.75 hence 3.73 gear ratio rather then saying a little less than 4 turns. Again, C4's method is correct IF THE WHEELS ARE LOCKED UP AND TURN THE SAME DIRECTION.

If your wheels spin in opposite directions, like an open differential or a G80 Eaton Gov Lok, then you will multiply the turns on the driveshaft by 2. Reason being, the spiders are spinning on the other side spider gear dividing your turns by 2. (This of course with 1 wheel jacked up and 1 on the ground)
So for an easy one to start with, IF your driveshaft turns a hair over 2 times, (to be exact, it would be 2.05 times) So 2 turns, x2 = 4.00 so safe to assume 4.10 ratio (4.11 if that is the ratio your rear end uses). If driveshaft turns a haiir over 1 1/2 turns (1.54 to be exact) x2 = 3 and safe to assume 3.08 gear ratio. driveshaft turns little under 1 3/4 times, x2 = 3.50 save to assume 3.42 ratio. It's very very easy to calculate guys. If you don't get it, just jack it up count your turns, and pay close to attention to little under, or little over, post it up and I can tell you what you have. Never been wrong yet.

I just bought a 1978 c20 and my driveshaft turns ALMOST 2 times? What do you think the gearing would be? And only 1 wheel spins

 

[HotRodPC]

You're positive it's ALMOST 2 times, and not just OVER 2 times? If it's just under 2x, most likely 3.73's, if it's just over 2 times, then most likely 4.10's.

1.9 times on a peg legger will be 3.73's, 2.05 times would be 4.10's.

 

[78c20]

You're positive it's ALMOST 2 times, and not just OVER 2 times? If it's just under 2x, most likely 3.73's, if it's just over 2 times, then most likely 4.10's.

1.9 times on a peg legger will be 3.73's, 2.05 times would be 4.10's.

Even if it's just 1 wheel spinning? Everyone else is talking about the wheels going the same or different directions

 

[HotRodPC]

Even if it's just 1 wheel spinning? Everyone else is talking about the wheels going the same or different directions

Yes, With one wheel on the ground, the other wheel in the air spinning, you would multiply by 2 to get your gear ratio on a peg legger. This is because the spiders are spinning in the opposite direction on the stationary axle splines.

So if it's a hair over 2 full turns, as in 2.05 turns x 2 = 4.10. If it's a hair under 2 turns as in 1.9 times or to be exact 1.865 turns x 2 = 3.73. If it's a hair over 1.5 turns or to be exact 1.54 turns x 2 = 3.08. Pretty simple math really.

On a locker, both wheels will need to be in the air, and both wheels will spin together so when you spin the wheels and no spiders to be absorbing any of the spin, then the count is exact and no need to multiply by 2.

 

[78c20]

Yes, With one wheel on the ground, the other wheel in the air spinning, you would multiply by 2 to get your gear ratio on a peg legger. This is because the spiders are spinning in the opposite direction on the stationary axle splines.

So if it's a hair over 2 full turns, as in 2.05 turns x 2 = 4.10. If it's a hair under 2 turns as in 1.9 times or to be exact 1.865 turns x 2 = 3.73. If it's a hair over 1.5 turns or to be exact 1.54 turns x 2 = 3.08. Pretty simple math really.

On a locker, both wheels will need to be in the air, and both wheels will spin together so when you spin the wheels and no spiders to be absorbing any of the spin, then the count is exact and no need to multiply by 2.

I did have both in the air to be sure. Awesome that really helped thanks. I'll use this in the future for sure

 

[HotRodPC]

I did have both in the air to be sure. Awesome that really helped thanks. I'll use this in the future for sure

So if you had both in the air, unless both were spinning in the same direction like with a locker, then your measurement is accurate. The one wheel needs to stay stationary so you can get an accurate count and multiply x 2.

 

[norman baker]

hello everyone, im new here and i have a newbie question. So i recently picked up a 77 chevy 3/4 ton 4x4 and was wondering how to tell the gear ratio. the guy i bought it from said they were stock but who knows. also, how do you check to see if you have a locker/limited slip/posi? sorry for such beginner questions but id like to learn more before i start handing out cash for other parts.

in some cases under the front seat is also build sheet, but in your case , body swap will be a problem, not sure if running the serial number works except for basics. gmheritedge .