gears q? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

useRc10 · 09-23-2002, 12:29 PM

i was thinking of changing the gears on my truck but i dont really know what to get and everything...its a '72 chevy lwb 350 with 3 spd manual...

what gear ratio would you recommend? where is a good place to get them? how much is a good price? what all is involved in cahnging them over?

i dont know what i have now but i know its not posi cause i'm only spinning 1 tire...thanks, -Nick

Michael A. K. G · 09-23-2002, 01:21 PM

A worn out Posi behaves EXACTLY like an open diff. You just have to replace the clutch packs. That's much cheaper (and considerably easier in many cases) than swapping in a Posi diff.

The most reliable way to tell if you have a Posi or not is to remove the diff cover and see if you can find a preload spring in the center of the diff.

Clutch-type Posis don't last forever. On a 30+ year old truck it is quite likely to be dead.

There are other types of Posis, but the clutch ones are common.

useRc10 · 09-23-2002, 01:24 PM

how do i tell what gears i have by counting teeth?
maybe eventually i'll get around to pulling the diff. cover but being at school only gives me a little bit of time to work on it on the weekend...thanks

Michael A. K. G · 09-23-2002, 01:31 PM

The gear ratio is the number of ring gear teeth divided by the number of pinion gear teeth. For instance, my Ford 7.5 has 41 ring gear teeth and 11 pinion teeth, for a gear ratio of 3.73.

For the ring gear, it helps to mark the starting point with liquid paper (on the side of the gear, not on the teeth), so you know where to stop. Dried liquid paper comes right off with brake system cleaner.

You can get an approximate count for an OPEN diff by lifting one wheel and putting the tranny in neutral. The gear ratio is the number of driveshaft turns per wheel turn MULTIPLIED BY TWO. For a working Posi diff, you lift BOTH wheels and forget about the factor of two.

You will have to count driveshaft turns down to 1/8 turn or better for this to work. And you have to know the available gear ratios.

Jmac05 · 09-23-2002, 01:44 PM

I am going to have the rear diff changed in mine within the next few weeks. I believe I have a 4.1 right now, and am sucking gas like an old man's boxers. I have about 100 mile a day commute, and I lose ~ 1/4 of a tank a day. I am debating on whether to go to a 3.07, or maybe around 3.7. anyone have any ideas on the mpg savings between the two? I have gotten used to making punk kids in rice burners look stupid at a stop light, but I don't really tow anything, so the mpg saving would be important.

MITCH1914 · 09-23-2002, 01:48 PM

so if a truck has an open diff only one tire turns at all times?
what you notice that some times both tires turn? in my case when i tap the pedal both of my tires chirp and they both leave a little rubber. but when i step on the peddal only one tire turns hard. i have been under the impression that i have an open differntial what is the deal?

did 68 c-10's come with posi traction? if so i will just rebuild my unit instead of buying a new one

09-23-2002, 12:29 PM	#1
useRc10 Registered User Join Date: Feb 2001 Location: Mechanicsville, VA, 23111 Posts: 1,138	gears q? i was thinking of changing the gears on my truck but i dont really know what to get and everything...its a '72 chevy lwb 350 with 3 spd manual... what gear ratio would you recommend? where is a good place to get them? how much is a good price? what all is involved in cahnging them over? i dont know what i have now but i know its not posi cause i'm only spinning 1 tire...thanks, -Nick __________________ -'02 Chevy Avalanche, 5.3L, 4x4... needing a small lift and bigger tires -'72 Chevy C-10(350, bored 60 over)dual flowmasters, 3 on the tree -'85 K5 Blazer,305 NO MORE...350 is in,2" lift,33" Mickey Thompson Baja Claw Radials -dads '70 burb (red, 5-7 drop, 350...) www.geocities.com/userc10/truckhome.html AIM: Auntjoedadda

09-23-2002, 01:24 PM	#3
useRc10 Registered User Join Date: Feb 2001 Location: Mechanicsville, VA, 23111 Posts: 1,138	how do i tell what gears i have by counting teeth? maybe eventually i'll get around to pulling the diff. cover but being at school only gives me a little bit of time to work on it on the weekend...thanks __________________ -'02 Chevy Avalanche, 5.3L, 4x4... needing a small lift and bigger tires -'72 Chevy C-10(350, bored 60 over)dual flowmasters, 3 on the tree -'85 K5 Blazer,305 NO MORE...350 is in,2" lift,33" Mickey Thompson Baja Claw Radials -dads '70 burb (red, 5-7 drop, 350...) www.geocities.com/userc10/truckhome.html AIM: Auntjoedadda

09-23-2002, 01:44 PM	#5
Jmac05 L.E.D. Tail lights, Baby! Join Date: Jan 2002 Location: Corona, Ca Posts: 290	GEARS I am going to have the rear diff changed in mine within the next few weeks. I believe I have a 4.1 right now, and am sucking gas like an old man's boxers. I have about 100 mile a day commute, and I lose ~ 1/4 of a tank a day. I am debating on whether to go to a 3.07, or maybe around 3.7. anyone have any ideas on the mpg savings between the two? I have gotten used to making punk kids in rice burners look stupid at a stop light, but I don't really tow anything, so the mpg saving would be important. __________________ *Prov 3:5-8* "Hook 'em" L.E.D. Taillights for sale NOW!!! If interested, visit: www.earlyclassic.com or our website: www.jmcled.com All 67-72 GM Taillights are on sale untill 5 p.m. on Dec 15th, with Guaranteed Christmas delivery! joel.mcwaters@jmcled.com

09-23-2002, 01:21 PM	#2
Michael A. K. G Registered User Join Date: Jul 2002 Location: California central coast Posts: 126	A worn out Posi behaves EXACTLY like an open diff. You just have to replace the clutch packs. That's much cheaper (and considerably easier in many cases) than swapping in a Posi diff. The most reliable way to tell if you have a Posi or not is to remove the diff cover and see if you can find a preload spring in the center of the diff. Clutch-type Posis don't last forever. On a 30+ year old truck it is quite likely to be dead. There are other types of Posis, but the clutch ones are common.

09-23-2002, 01:31 PM	#4
Michael A. K. G Registered User Join Date: Jul 2002 Location: California central coast Posts: 126	The gear ratio is the number of ring gear teeth divided by the number of pinion gear teeth. For instance, my Ford 7.5 has 41 ring gear teeth and 11 pinion teeth, for a gear ratio of 3.73. For the ring gear, it helps to mark the starting point with liquid paper (on the side of the gear, not on the teeth), so you know where to stop. Dried liquid paper comes right off with brake system cleaner. You can get an approximate count for an OPEN diff by lifting one wheel and putting the tranny in neutral. The gear ratio is the number of driveshaft turns per wheel turn MULTIPLIED BY TWO. For a working Posi diff, you lift BOTH wheels and forget about the factor of two. You will have to count driveshaft turns down to 1/8 turn or better for this to work. And you have to know the available gear ratios.

09-23-2002, 01:48 PM	#6
MITCH1914 Registered User Join Date: Mar 2001 Location: indianapolis,indiana,usa Posts: 506	so if a truck has an open diff only one tire turns at all times? what you notice that some times both tires turn? in my case when i tap the pedal both of my tires chirp and they both leave a little rubber. but when i step on the peddal only one tire turns hard. i have been under the impression that i have an open differntial what is the deal? did 68 c-10's come with posi traction? if so i will just rebuild my unit instead of buying a new one



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