Read-End Ratio?

[JoeyGowdy]

Just curious, the 2070741 is the 8.75 (8-3/4) rear-end correct?

What would be the driveshaft to read-end ratio?

I'm trying to find the right gearing to make the speedometer work properly.

[A383Wing]

Yes, the 741 case is a 8-3/4" rear end....you need to find out the gear ratio by counting the ring teeth and then divide that by the number of pinion teeth....

[Ghoste]

It is an 8.75 rearend but that's a casting number and you need to establish the gear ratio.  The surest way is to remove it and check but if you mark the driveshaft and a wheel and calculate the number of turns of the driveshaft to get one full revolution of the wheel you'll be close.  From there we can help you with the speedo pinion.

[A383Wing]

Here is chart in this thread

http://www.dodgecharger.com/forum/index.php/topic,50317.0.html

[JoeyGowdy]

Ka'Pla! Success!! Found it - had to use the wire-brush, but it was still there... up-side-down I might add... reads 2.76 which isn't to hot is it?

[Ghoste]

It a highway cruising gear, yes.

[JoeyGowdy]

It a highway cruising gear, yes.

True... 15mpg  then again, it can eaisly do burnouts... perhaps it has been rebuilt and/or replaced in it's past with a better one?  :?:

[Ghoste]

Is this 2.76 stamped on the ring gear or on a tag attached to the outside of the housing?

[A383Wing]

Bet it's stamped in a flat pad below the fill plug....I might have a different set of gears in it ....could have been changed.....

[JoeyGowdy]

Looks like the original tag (flat piece on the right), here's a photo from my cell phone taken a couple minutes ago (didn't see any other tags or stamps - could be original or they might not have placed another tag on it if it's been changed in it's past):

[A383Wing]

Just because the tag says 2.76, doesn't mean that's what is inside....you need to count how many times the driveshaft goes around when the tire makes 1 complete revolution. Do this on jackstands and with the engine off.

[Ghoste]

I agree with Bryan.  I thought for a bit there you were reading it off of the ring gear but you can't rely on the stamped tag.  It could very well be a 2.76 but it may not be either.

[JoeyGowdy]

A'ight, just measured it - the driveshaft turns 1-3/4 (1.75) everytime ONE wheel makes a complete turn... thoughts?

[Hemidog]

That doesnt sound right? did you count correctly?

[JoeyGowdy]

That doesnt sound right? did you count correctly?

Yes, one full rotation of the wheel turned the driveshaft 1-3/4 (1.75) times.

I only had the driver's side wheel jacked-up... the passanger side was still on the ground.

Perhaps I should try to rotate the wheel two full turns and see what happens?

[FLG]

Its an open rear...therefore you need to double that number 

On a posi rear both tires rotate so only one revoultion is needed. On an open rear since only 1 wheel turns you need to either rotate the tire twice or just double the reading when you rotate it once.

Hence why the tires spins so damn fast if you power brake a car with an open diff

[JoeyGowdy]

Its an open rear...therefore you need to double that number 

So doubling the 1-3/4 would probably make it about a 3.50 or 3.55 ?

[John_Kunkel]

Now that you know the gear ratio you need the tire diameter to determine the proper speedo pinion.

[JoeyGowdy]

Now that you know the gear ratio you need the tire diameter to determine the proper speedo pinion.

Front: 225/70/R14

Rear: 245/70/R15

[A383Wing]

He did not say "Tire Size"....he said "Tire Diameter"....you must measure the diameter with a tape measure.

Then get the chart....

[FLG]

245/70-15's have a diameter of 28.5"

[JoeyGowdy]

Just turned the driver side wheel twice and the driveshaft turned 2-3/4 (2.75) times... the passenger side is still on the ground.

So... 2.76 or 3.5X ?

[FLG]

Weather you turned the wheel twice or turned it once and doubled the number they should be the same. Either double and triple check or pull the carrier and start counting teeth.

If your supposed to have 2.76 and your saying you checked and it takes 2.75 revolutions id be pretty sure it is a 2.76 gear ratio,

[JoeyGowdy]

Eh, knowing my luck I'll probably have to pull it and check... one wheel revolution turns the driveshaft 1-3/4 (1.75)... two wheel revolutions turns the driveshaft 2-3/4 (2.75)... both times I was turning the driverside wheel and had the passengerside wheel on the ground so it wouldn't move.

[A383Wing]

Here's another way....put both jack stands under the rear...have both rear tires off the ground....make a chalk mark on both tires at the bottom, and on the drive shaft yoke.....now you are going to lay on a creeper, or the ground, and slowly spin both tires (one at each hand) the same direction, keep an eye on the drive shaft and count how many times it goes around when both of your chalk marks on the tires end up at the same place you started.