posi rear end gears

[ivancito]

I am unsure what information I might need to order proper gears.

My truck is a 1968 c-10, it currently has a limited slip differential but I am looking to get posi back there.

Guess the question I have is what information will be needed and will posi gears screw with the speedometer accuracy?

Information I do know

12 bolt rear w/ current limited slip, I forget the ratio we determined.

Any help will be appreciated.

[LongBox]

Do you mean you want to go from limited slip (positraction in GM speak) to a locker?

Either way, if you don't change your gear ratio, your speedo drive will not need to be changed.

[ivancito]

Thanks for the quick response.

I did not know limited slip and posi were interchangeable.

I will have to venture a guess that I don't have limited slip gears since that's the case. I know for a fact that only one tire spins when given enough persuasion.

I hope to get the gear ratio I have soon, just need to find time. If I remember correctly finding the gear ratio is simple enough.

[Bigdav160]

What you mean is you have an open differential. Posi-traction is a brand name for a limited slip differential.

No need to change the gears just the differential case.

[ivancito]

Apparently I have 3.08 gears.

What gears would you recommend for a sbc 350 paired with a monster 700 r4 transmission. Just feels like the truck has more to give but seems to be held up somewhere.

Transmission info:
MCP-700R4 700R4 HD Transmission Master Conversion Package 2WD
Torque Converter: 1650 Stall
TV Cable Items: Universal Fit Throttle Cable with metric bolt, Chrome Throttle Cable Bracket
Speedometer Items: Speedometer Driven Gear, Housing, Support Bracket & O-Ring
Included Accessories: Fillertube & Dipstick, Linkage Nut, 2 Brass Coolant Line Fittings, Metric Converter Bolts, Univ Trans Mount w/bolts, External Lock-Up Kit, Trans Cooler Kit, Univ Dust Cover

[Mike C]

3.73 are a pretty good match with the 700R4

[ivancito]

thanks for the replies.

if I went with 3.73 gears would I need a change for speedometer to remain correct?

[coreyjhen]

You can figure out which speedometer gears you need here: http://www.tciauto.com/tc/speedometer-gear-calculator/

Instructions for changing the gears are available here: http://www.73-87.com/7387garage/drivetrain/spdgears.htm

It's a relatively easy change.

[ivancito]

Originally Posted by coreyjhen

You can figure out which speedometer gears you need here: http://www.tciauto.com/tc/speedometer-gear-calculator/

Instructions for changing the gears are available here: http://www.73-87.com/7387garage/drivetrain/spdgears.htm

It's a relatively easy change.

Thanks for the links, I need all the help I can get.

[LongBox]

One thing is that I would NOT go to a 700R4 with 3.08 rear gears. Once the 700R4 shifts into 4th (OD), you would probably be turning around 1400 RPM on the highway at 60 mph, depending on your tire size.

My 98 has 3.73 and the 4L60E, which is a 700R4 with electronic controls, and it turns about 2200 rpm or so at 60. Gets reasonable mileage as well.

Back to the issue here, I think if you are not experienced with working with rear axles and diffs, it would be a good idea to find a specialty shop to make the changes you want. Going to 3.73 and positraction would make a great improvement, especially if you get the 700R4 installed. I'd get it all done by one competent shop and at the same time.

[TwoFiftyShifter]

Originally Posted by LongBox

One thing is that I would NOT go to a 700R4 with 3.08 rear gears. Once the 700R4 shifts into 4th (OD), you would probably be turning around 1400 RPM on the highway at 60 mph, depending on your tire size.

My 98 has 3.73 and the 4L60E, which is a 700R4 with electronic controls, and it turns about 2200 rpm or so at 60. Gets reasonable mileage as well.

Back to the issue here, I think if you are not experienced with working with rear axles and diffs, it would be a good idea to find a specialty shop to make the changes you want. Going to 3.73 and positraction would make a great improvement, especially if you get the 700R4 installed. I'd get it all done by one competent shop and at the same time.

2200 at 60? Probably more like 1800-1900.

[demian5]

I am pulling out a bad 373 r&P with a good posi and putting in a 3.08. Too bad you are so far away, you could have this posi for next to nothing.

[Timthetoolman]

I have a similar question. I have the 373 gears but would like to make both tires turn if I bump the gas.
The mechanic doing the rebuild said the cheaper locker would be easy but said the more expensive one would be harder to install. He seemed a little shy about it so maybe he's not familiar with them.
1. Which one would I get for a crate 350 and 700R4 combo that would keep both wheels spinning in rain or 'giving it the gas'?
2. Is this a difficult install, should I take it to a specialist?

Thanks
Tim

[truckster]

Originally Posted by Timthetoolman

Which one would I get for a crate 350 and 700R4 combo that would keep both wheels spinning in rain or 'giving it the gas'?

No offense intended, but why do you want both wheels spinning in the rain? That's a really good way to end up in a ditch.

I like posi - both my Blazer and my Z-28 have posi rear ends, but you need to be very judicious when driving on slick pavement. Even on dry pavement the Z-28 will end up going the wrong way if I'm not careful.

[Mike C]

Eaton limited slip. You will get 100% more traction BEFORE the tires start spinning and at that point loss of control could occur so back off the gas!

If you have to ask that question on the internets then DEFINITELY take it to someone who knows what they are doing. Swapping in a limited slip is not that difficult if you are NOT changing the gears at the same time.

But expect to pay $500 for the posi, $100 for a basic install kit with shims and carrier bearings, and if I was doing the work I would charge three hours labor. Jacking it up, pulling the tires, pulling the axles, measuring backlash, removing the carrier and ring gear, pressing on new side bearings, shimming the case and measuring/adjusting backlash to match. Reinstalling axles, sealing cover and refilling diff.

And as an owner, it would be worthwhile to pay for axle seals and a new pinion seal as well while they were in there. So make sure a new pinion nut and ring gear bolts are in your install kit. So maybe even 4 hours labor.