Rear gear ratio?

[manimal]

How do you check what gears you have without pulling the rear diff cover?

[cdowns]

jack the rear end up put a piece of tape on pass side wheel have in neutral turn driveshaft count the number of revolutions of drive shaft to one revoulation of rear wheel will give you a ball park figure of your ratio

[manimal]

Well I did that. And the driveshaft turned '5' times to the wheels one turn!! Could this be right? Its a 1/2 ton that originally had a 6 cyl and three on the tree.

[Elsie72]

If the rear end doesn't have a posi, if only 1 tire turns with the above test, the spider gears will cause the opposite wheel to turn twice as fast.

Ensure that either both wheels are turning through the whole test, or block one wheel, and devide the result by 2.

[manimal]

well I did that(block one wheel) and now I come up with 1 3/4 turn.nobody here to help me with both wheels. Guess I need to pull the diff cover

[Longhorn Man]

pull the cover off, if you're lucky, the oil is only 20 years old and needing changed. It's possably original oil and only doing 50% of it's job.

[msgross]

Originally Posted by Longhorn Man

pull the cover off, if you're lucky, the oil is only 20 years old and needing changed. It's possably original oil and only doing 50% of it's job.

I agree...can always use new gear lube.... there is a post on here that helps you decipher the ring gear numbers.

[cdowns]

of the hundreds upon hundreds of rears i've checked in the past 40 years i've never seen one spin the tire at twice the axel ratio

[msgross]

Originally Posted by cdowns

of the hundreds upon hundreds of rears i've checked in the past 40 years i've never seen one spin the tire at twice the axel ratio

i've checked out a lot of rears in my time too, i've seen them bounce more than spin ...oh wait, you mean trucks

[LockDoc]

-
Here are the numbers you will be looking for on the ring gear & Pinion when you get the cover off.....

Ratio 2.56:1 - Tooth Combination 41,16
Ratio 2.73:1 - Tooth Combination 41,15
Ratio 3.07:1 - Tooth Combination 43,14
Ratio 3.08:1 - Tooth Combination 40,13
Ratio 3.08:1 - Tooth Combination 37,12
Ratio 3.31:1 - Tooth Combination 43,13
Ratio 3.36:1 - Tooth Combination 37,11
Ratio 3.42:1 - Tooth Combination 41,12
Ratio 3.55:1 - Tooth Combination 39,11
Ratio 3.73:1 - Tooth Combination 41,11
Ratio 4.10:1 - Tooth Combination 41,10
Ratio 4.11:1 - Tooth Combination 37,9

LockDoc

[Rod]

You could also assume ratios based on glove box stickers with no listed gear option changes. You just go by standard equipment lists depending on the tranny and engine size. Typically straight 6 came with 3.73 and v8 came with 3.07 on 2wd model in 1/2 ton series in the 67-72 era.
You could also use the code stamped on the axle tube also...hoping it had not been swapped or altered.

[69TowRig]

Originally Posted by cdowns

of the hundreds upon hundreds of rears i've checked in the past 40 years i've never seen one spin the tire at twice the axel ratio

Regardless the above advice is true, if you hold one wheel and turn the driveshaft the other wheel turns at twice the ratio. This has to do with the fact that the carrier walks the spider gears around the side gear that is stationary from the held axle shaft. Those gears in turn drive the opposite side gear one extra turn for each revolution of the carrier.

It's a fun math problem to do when you are a Mechanical Engineering student (I know because it was a homework problem I had to do many years ago).