Gear ratio?

[Zonaman]

I've got a posi-traction rear end, that's all I know. How can I determine the gear ratio on this, preferably without taking things apart?

Thanks.

[72moneypit]

If you have a tachometer, you can calculate your gear ratio using MPH, RPMs and rear tire height. I don't remember the exact calc, but there are several places on the internet that have it. If you're like me and hit 4000 RPMs at 75 MPH, you've probably got 4.11's

[Jeepster376]

Originally Posted by 72moneypit

If you have a tachometer, you can calculate your gear ratio using MPH, RPMs and rear tire height.

With an automatic trans it's not so easy to calculate that way, since the torque converter is always slipping by a couple to several hundred RPMs it's hard to nail it down with a calculation. Also speedometer error could affect this method of calculating.

[Jeepster376]

Just jack up the rear, put it in neutral, mark a tire and the driveshaft with chalk, rotate the driveshaft with your hands until the tire makes one revolution and count how many times the driveshaft rotates. You can make a pretty good guess from there. If it turns just over three turns you've got 3.07s, if it's about 3 3/4 it's 3.73, if it's just over 4 you've got 4.11s

[Jeepster376]

BTW, this only works for a posi rear. If you, like me, do not have a posi, jack up one wheel only and rotate the tire 2 revolutions, all else is the same.

[Longhorn Man]

popping off the cover is a good way, and not very hard at all.
When was your diff oil chainged last?

[Zonaman]

Couldn't say. What'll I see in there that'll tell what it is?

Thanks.

[Longhorn Man]

You'll see the gears in there. All you need to do is count the teeth on the drive gear, and on the driven gear, and devide the two. If the gears are stock, then the tooth count will be stamped somewhere on there. I'll see if I can't dig up a pic somewhere.
Also, if your truck is all original, or a well preserved truck, there are some asumptions that can be made too. What year is your truck? What is the combo your truck came with originally...as in 1/2 ton or 3/4 ton, what size engine and kind of tranny...and is there any gear option listed in the glove box?

Regardles of what we say, was stock, they could have been chainged out at any point in the past 32 or more years your truck has been around.

[71Rat]

i tryed the way jeepster 376 said, and when i did mine i came out with just under 2 revolutions of the driveshaft... then i talked to a few guys and found out your supposed to multiply that by 2..... not 100% sure if that is correct but i never heard of gm making a rear thats under a 2.56:1 and with a 3 speed saginaw i couldnt get over 65-70 mph with 28.5 inch tall tires... so if i was you i would pull off the cover and check it out and then replace the fluid... but if you have a posi remember that some/most posi's require a certain type of fluid or an additive.

Rat

[Jeepster376]

Originally Posted by 71Rat

i tryed the way jeepster 376 said, and when i did mine i came out with just under 2 revolutions of the driveshaft...

Do you have an open (non posi) diff? If so did you see my additional post that for an open diff you need to leave one tire on the ground, raise only one tire and make it go round twice. This is because with an open diff your tires will tend to turn at different rates if they even go in the same direction so it's not possible to track the revolutions of your carier, unless you anchor one tire turn the other one twice this will rotate the carrier by one turn. With a Posi it's easy because everything turns together.