A couple of gearing questions

[TugOwar]

Good day all,

1) A few days ago I had the back of my 68 3/4 GMC off the ground adjusting the rear brakes and decided to clean up the little tags on the 10 bolt cover and see what they said. One tag bolted on the right side say "4:10", I know what that means.

But the tag bolted on the left says "use limited slip gear lube only." Now I know that limited slip is different from posi traction but I don't recall seeing "posi-trac" gear lube in the parts store. So I'm wondering if just because it calls for limited slip lube does that necessarily mean it's a limited slip rear end, or does a posi-trac also take limited slip lube? I don't know if it means any thing but I noticed while adjusting the brakes that if I turn one wheel, the other wheel turns in the same direction, forward or back.

I hope all that isn't too confusing. From what I've written is it possible to tell if I have limited or posi rear?

2) I know this question has been asked a blue zillion times but for whatever reason I can never find anything using the search function, I don't know what I'm doing wrong, anyay..Please forgive the repetitive question.

My speedometer is wrong, too fast actually. I think I remember reading that this is because one of the speedo gears in the tranny needs to be changed, either the drive gear or the one that slides into the tranny. But I don't which one, and I don't know how to determine which to replace it with or where to get a replacement.

Here is what I know:
4:10 gears
10.50x31x15 rear tires
TH350 tranny
Factory Speedometer

What other info do I need to get my speedo correct? Any help would be greatly appreciated.

Thanks,
Tug

[1969 GMC]

Limited slips and posi tractions are one in the same, so the additive will work for you. I am guessing you have a dana 60 rear axle, too.

[bigjimzlll]

limited slip======posi

[TugOwar]

Thanks for the info gentlemen. I always thought limited and posi were two different things,,,learn something new every day.

What about my speedo? Anything I can do about that?

[Fred T]

The speedo gear is on the left rear of the tranny, has a cable running into it. First you need to figure out how far off it is. Drive down the highway at a constant 60 mph on the speedo. Clock yourself between mile markers. How many seconds does it take? Divide 3600 by the number of seconds it took to travel the mile.
3600/30 = 120 mph
3600/60 = 60 mph
3600/ 90 = 40 mph

Next, remove your speedo gear. Jack up the rear of the vehicle, so most of the tranny fluid goes to the front of the tranny. Have a drain pan to catch the rest. Remove the cable, it unscrews from the fitting. There should be a bolt holding the fitting in the tranny, remove the bolt and the fitting. This is where the fluid will run out and down your arm. Inside is a plastic gear with a shaft on one end, the speedo gear.

Take the gear and the fitting to a tranny shop, preferably one that sells auto tranny parts. Tell them your speedo reads 60 mph when you are really going say 45. They should be able to sell you the proper gear for a few bucks. Also, get the inner and outer o-rings for the fitting.

When you assemble the parts, use petroleum jelly to lube the o-rings. Put the fitting in and connect the cable. After the truck is back on level ground, add some fluid, then check again after the vehicle is warm. You will probably have to add some more fluid.

[TugOwar]

Thanks very much Fred, that sounds easy enough, even for me!