How to tell rearend ratio?

[Dodgerdallas]

Ive got mine installed and have a parts car I need to determine the gear ratio,whats the easiest way?

[c00nhunterjoe]

If its still factory there should be a small tag on one of the 3rd member bolts with the ratio stamped on it

[Dodgerdallas]

I'll look thanks-what if the tags not there?Ive done it the old fashion way of jacking up rear end and count the yolk revolutions as th wheel makes a complete turn,that can give you a rough estimate.

[c00nhunterjoe]

The other option is remove the pumpkin. The ring gear is stamped.

[404NOTFOUND]

One last option. Take a drive on the highway. If your car seems ordinary, you have 308s. If you can hear a good roar, you have 323s. If gas mileage and engine wear starts to concern you, you have 355s. If driving on the highway is a head exploding nightmare, you have 410s. If you simply can't go there, you have 456s.

[Cooter]

Pull the chunk out and look. Counting revolutions doesn't work if you don't know how to do this. Gear ratio's were figured on 26" tall tires, if you have 28" tall, can throw off your count.

[Fred]

The best and acurate way to find out the gear in the diff is to count the number of teeth on the ring gear and divide it by the number of teeth on the pinion gear.
That will give you the exact ratio. 

[bobs66440]

If it's an open rear, Jack one rear wheel off the ground and turn it exactly TWO revolutions. Count how many times the driveshaft turns. That number is your axle ratio.

If it's a sure grip, BOTH wheels need to be off the ground. Turn the tire ONE revolution. Count how many times the driveshaft turns. That number is your axle ratio.

It doesn't matter what tire size you have. One revolution is one revolution regardless. Tire size only matters when the car is on the ground and rolling. Then the RPMs change for a given speed. Taller tire gives the same effect as a higher gear ratio (lower numerically). 

[Dodgerdallas]

So it's turning the tire 2 revolutions instead of one? Ok I didn't remember correctly,thanks I can do that quickly when I get home...

[resq302]

Bob,

Curious as to why an open rear gets TWO full turns while a sure grip gets only ONE turn?

[Cooter]

It doesn't matter what tire size you have. One revolution is one revolution regardless. Tire size only matters when the car is on the ground and rolling. Then the RPMs change for a given speed. Taller tire gives the same effect as a higher gear ratio (lower numerically).  

Wrong.....A taller tire will throw off the amount of turns at the pinion. Not by much mind you, but it will. Otherwise, if it made no difference, your speedo would stay the same whether you had a 31" tall tire, or a 26" tall tire. If this were true, a transmission wouldn't need larger gears with more teeth on 'em they'd all be the same size.

[Cooter]

Bob,

Curious as to why an open rear gets TWO full turns while a sure grip gets only ONE turn?

I would assume it Has to do with the spider gears. Your walking around the gears. Like I was saying, different gear ratios/tire sizes effect the outcome of "Guessing" the ratio, but you guys go ahead and try it.

[bobs66440]

.

[bobs66440]

It doesn't matter what tire size you have. One revolution is one revolution regardless. Tire size only matters when the car is on the ground and rolling. Then the RPMs change for a given speed. Taller tire gives the same effect as a higher gear ratio (lower numerically).  

Wrong.....A taller tire will throw off the amount of turns at the pinion. Not by much mind you, but it will. Otherwise, if it made no difference, your speedo would stay the same whether you had a 31" tall tire, or a 26" tall tire. If this were true, a transmission wouldn't need larger gears with more teeth on 'em they'd all be the same size.

Yes, a taller tire will make a difference when the car is on the ground and rolling. But, when the tire is in the air, the pinion doesn't know or care what is bolted to the end of the axle. Think about it. You could have a cement block bolted to it, it doesn't matter...one revolution is one revolution. A larger tire will turn more slowly (while driving on the road) for a given road speed compared to a smaller tire. That's why you need to change the trans gear to compensate. Say you have a 10' tall tire and you travel 100' at 5mph. That tire will only turn, say, once. But if you were riding on the brake drum and go the same 100' at 5mph it would turn like 50 times right? You would then need to change the gear in the trans to make the speedo read the same 5mph. Now, if you raise the car off the ground in both cases and turn the tire or brake drum ONE revolution, the axle will also turn only ONE revolution. And the ring gear, pinion, driveshaft, speedo, will respond to only ONE revolution.

[Ghoste]

Tire size will affect the distance the car rolls when it is on the ground for each revolution but when the tire is in the air it won't matter if it is twelve inches or twelve feet in diameter- one revolution will always equal one revolution.  The speedometer drive pinion is being used to calculate a speed which is based on distance travelled for those revolutions so yes, it has to be changed for a different tire size as the tire size changes the distance travelled per tire revolution.  

[bobs66440]

Tire size will affect the distance the car rolls when it is on the ground for each revolution but when the tire is in the air it won't matter if it is twelve inches or twelve feet in diameter- one revolution will always equal one revolution.  The speedometer drive pinion is being used to calculate a speed which is based on distance travelled for those revolutions so yes, it has to be changed for a different tire size as the tire size changes the distance travlled per tire revolution.  

Exactly, thank you.

[John_Kunkel]

Jeez, the guy asks what time it is and you guys tell him how to build a watch    bobs66440  gave all the info needed to answer the OP's question.

[Fred]

Tire size will affect the distance the car rolls when it is on the ground for each revolution but when the tire is in the air it won't matter if it is twelve inches or twelve feet in diameter- one revolution will always equal one revolution.  The speedometer drive pinion is being used to calculate a speed which is based on distance travelled for those revolutions so yes, it has to be changed for a different tire size as the tire size changes the distance travlled per tire revolution. 

Couldn't have said it better myself.

[Fred]

Ive got mine installed and have a parts car I need to determine the gear ratio,whats the easiest way?

The best and acurate way to find out the gear in the diff is to count the number of teeth on the ring gear and divide it by the number of teeth on the pinion gear.
That will give you the exact ratio. 

[440 Fanatic]

One last option. Take a drive on the highway. If your car seems ordinary, you have 308s. If you can hear a good roar, you have 323s. If gas mileage and engine wear starts to concern you, you have 355s. If driving on the highway is a head exploding nightmare, you have 410s. If you simply can't go there, you have 456s.

You left one out:
If you feel like you don't even need 3rd gear until 60+ mph , you have 2.76s