Proper way to check Rear Diff Gear Ratio??

[Yamariv]

Finally go around to doing my rear diff oil change today and did the tire spin while counting the axle spins to try and figure out my diff gearing.

I tried the one full rotation of the rear tire and counted my drive shaft rotations which didn't even get around twice which I thought was weird. I then googled a video and one guy said you have to rotate the rear tire 2 full turns to find your gear ratio which seemed to work much better. By using the 2 full turn method, my drive shaft rotated 3 full turns plus a sliver. I'd guess that to be 3.08 or 3.10 gearing. Is the two full rotation the proper way of doing it?

[Palf70Step]

That sounds right. On gm 12 or 10 bolts. I like to pull the rear cover, clean it out and check the numbers on the ring gear. Then seal it up and put fresh 90w in it. I always figure, that lots of folks just run them till they break. So I like to do the service Nd visual on the diff.

[Ironangel]

Originally Posted by Yamariv

Finally go around to doing my rear diff oil change today and did the tire spin while counting the axle spins to try and figure out my diff gearing.

I tried the one full rotation of the rear tire and counted my drive shaft rotations which didn't even get around twice which I thought was weird. I then googled a video and one guy said you have to rotate the rear tire 2 full turns to find your gear ratio which seemed to work much better. By using the 2 full turn method, my drive shaft rotated 3 full turns plus a sliver. I'd guess that to be 3.08 or 3.10 gearing. Is the two full rotation the proper way of doing it?

For an open differential two spins is correct. For a locker or a limited slip jack both wheels and spin once. You have a 3:08 ratio...Next time you have the cover off you can read the numbers on the edge of the ring gear to get your ratio. My 3:73 has 41-11 stamped on the edge of the ring gear, 41 teeth on the ring gear and 11 teeth on the pinion. Yours is probably 40-13...

[LockDoc]

-
Here are the charts if you can find the code stamped on your housing or axle tube.

https://www.67-72chevytrucks.com/vbo...d.php?t=578793

Post #8 and #19 show a couple of locations to look for the code.

LockDoc

[Yamariv]

Dang, in my excitement to get the rear diff cleaned out and oil changed I totally forgot to look for numbers on the gears..Doh!

How big of a job would it be to put a different ratio, is this a big job as it bring it into a shop to have it done? Is it expensive?

[72SB]

Installing rear gears is not only a learned skill...but a practiced one as well. Depending on what gear ratio you want to change to, you may need a different carrier also. For example, 3:73 ratio will not work on the carrier size for 3:08 so you would have to get the carrier series for 3:73.

While I do my own wrenching, gears are something I have done at a shop. Labor is generally around $400.

If you change gears may as well go with a posi also since the carrier needs to come out

I have 3:73 peg leg but would like 3:08 posi

[OregonNed]

Look carefully for a shop to do this. I went to a supposed good shop for mine for rebuild and gearing change and it was totally botched. Looked to the local C-10 guys for recommendations and found an amazing mechanic to do mine. Definitely follow the break in suggestions/procedures for sure - I did and am very happy with the end results.

[Yamariv]

Originally Posted by OregonNed

Look carefully for a shop to do this. I went to a supposed good shop for mine for rebuild and gearing change and it was totally botched. Looked to the local C-10 guys for recommendations and found an amazing mechanic to do mine. Definitely follow the break in suggestions/procedures for sure - I did and am very happy with the end results.

What was the cost to have this done so I have a bit of an idea if it will be worth it or not. What gears did you go from?

[Palf70Step]

When I went from a 3.73 to 3.08 (and I had the ring and pinion) it was about $900. All new bearings and seals and all. IF I would have had to get a new carrier, you could add another 600

[Yamariv]

Originally Posted by Palf70Step

When I went from a 3.73 to 3.08 (and I had the ring and pinion) it was about $900. All new bearings and seals and all. IF I would have had to get a new carrier, you could add another 600

Oh wow, you went to a 3.08 howcome, just for highway?

Wow, thats quite the cost! Not sure if it would be worth it then in my case :S

[72SB]

Originally Posted by Palf70Step

When I went from a 3.73 to 3.08 (and I had the ring and pinion) it was about $900. All new bearings and seals and all. IF I would have had to get a new carrier, you could add another 600

Was this a truck 12 bolt 8.875" rear end?

I thought the truck 12 bolt with 3:73 had a 4 series truck carrier and for 3:08 you need a 3 series?

Do you have 3:08's that fit a 4 series carrier??

[Palf70Step]

Yes it was a 12 bolt rear. From what I was told it depends on the year. My 64's rear end it would take the 3.08 from a 72 rear (just remembered i had. Pinion ring and carrier from the 72 ) Although bearings and all were a 1 year only thing, so made it harder to find and of course pricier.

and yes, I did it for the highway cruising as I was hooked to a manual 3 speed. Down here you don't need lot of gear unless you are towing as it is pretty flat.

[72SB]

Yeah I do alot of Fwy driving so 3"73's need to stay about 60mph to keep it under 3k rpm on the I6

Since I want a Posi I would need a new carrier anyway so looking at this combo for posi and gears for my 72

Eaton Diff
https://www.summitracing.com/parts/etn-19587-010

gears
https://www.summitracing.com/parts/m...=true#overview

[OregonNed]

Originally Posted by Yamariv

What was the cost to have this done so I have a bit of an idea if it will be worth it or not. What gears did you go from?

My rebuild also included an upgrade from 3:08 open differential to a new POSI with 3:42 gears. The shop was just changing ownership from a long time owner to a new father and son business. The father apparently knew the owner for many years. I had my driveshaft balanced and at the owners recommendation I had the work all done as the pinion bearing seemed suspect, and I thought the upgrade would be good.

During the rebuild, he changed everything - new bearings, new gearing, one new axle, etc. He charged me about $1200 including tax. After the work, and when I drove it home, it whined horribly from the rearend. I promptly took it back to complain and to have him test drive it. He blamed all the noise on the transmission - and I knew it was OK. In the end, the new owner agreed to rebuild it once more at his expense to make it right. He claimed the ring gear was messed up because of initial excessive wear, and it not being installed correctly.

He did the rebuild and told me it "sounded better" - He claimed this by getting it on the freeway, and hammering it hard to make sure it was "broken in" - well, I attempted the recommended drives to break it in, and the howling was bad if not worse still.

At this point I didn't trust taking it back. I decided to eat the cost as a lesson learned, and found a local mechanic who knew what he was doing. I reinstalled the diff, and broke it in and it was quiet and running like it should be. The new mechanic found the bearings for the new POSI carrier extremely galled up, as well as the other bearings from jamming it in and out. So in the end I just went back with a new ring and pinion at 3:08, and kept the new POSI. Again, about $1000 I think this time - also the other axle replaced too. Extremely happy with the new mechanic!

Sorry for the long winded answer, but really suggest making sure the shop is up to the job if you have it done.

[Yamariv]

Originally Posted by OregonNed

My rebuild also included an upgrade from 3:08 open differential to a new POSI with 3:42 gears. The shop was just changing ownership from a long time owner to a new father and son business. The father apparently knew the owner for many years. I had my driveshaft balanced and at the owners recommendation I had the work all done as the pinion bearing seemed suspect, and I thought the upgrade would be good.

During the rebuild, he changed everything - new bearings, new gearing, one new axle, etc. He charged me about $1200 including tax. After the work, and when I drove it home, it whined horribly from the rearend. I promptly took it back to complain and to have him test drive it. He blamed all the noise on the transmission - and I knew it was OK. In the end, the new owner agreed to rebuild it once more at his expense to make it right. He claimed the ring gear was messed up because of initial excessive wear, and it not being installed correctly.

He did the rebuild and told me it "sounded better" - He claimed this by getting it on the freeway, and hammering it hard to make sure it was "broken in" - well, I attempted the recommended drives to break it in, and the howling was bad if not worse still.

At this point I didn't trust taking it back. I decided to eat the cost as a lesson learned, and found a local mechanic who knew what he was doing. I reinstalled the diff, and broke it in and it was quiet and running like it should be. The new mechanic found the bearings for the new POSI carrier extremely galled up, as well as the other bearings from jamming it in and out. So in the end I just went back with a new ring and pinion at 3:08, and kept the new POSI. Again, about $1000 I think this time - also the other axle replaced too. Extremely happy with the new mechanic!

Sorry for the long winded answer, but really suggest making sure the shop is up to the job if you have it done.

Ugh, that really sucks! Its so tough to know if you have a good mechanic before hand. I've had several stories like this with crappy mechanics over the years owning dirt bikes and snowmobiles and now I try to do any wrenching on my own. The differential stuff seems pretty complicated though so the 3.08 gears are gonna stay for a while lol

[72SB]

IMHO if having gears done, go to a "Driveline Shop"..not just a general mechanic.

I have used the same shop for 30 years doing 6 different cars of mine

I bring in the parts and they install. Anyone, including me, can bolt this stiff in...its setting up correct tolerances and knowing how to do that and having done it hundreds if not thousands of times that makes the difference of a quiet set vs a noisy mess that breaks parts.

[OregonNed]

Agree - I went to a well established driveline shop and got the shaft - look carefully!

[Already Gone]

What are you wanting to accomplish by changing the rear end ratio ?

[Yamariv]

Originally Posted by Already Gone

What are you wanting to accomplish by changing the rear end ratio ?

I'm probably not going to change it cause of the cost and difficulty. Ideally if it was easy, I'd like a higher rear end gear so I don't have to ride the clutch so much taking off in first.

[ghackett1]

Just found my code recently. HA = 3:73 non posi 1968 c10