Transmission help

[dliving69]

I have a Th350...So i was wondering if there is anyway you can add a overdrive? or possibly make it where it doesnt run so high on the RPM's going down the highway.

[Willys47]

look at Gearvenders They make over and under drives

[GMCJOE]

For the price you would spend on a Gearvendor's overdrive, I would just suggest purchasing a built 700r4. I thought about going that route with my TH400 till I seen the price of it. This is the company I am going thru to purchase my new transmission when the time come. They have several options when it comes to what HP rating you are wanting. Hope this helps.

http://www.transdepot.net/

You could also change the gear ratio out in your axle to drop your RPM's.

[dliving69]

Originally Posted by GMCJOE

For the price you would spend on a Gearvendor's overdrive, I would just suggest purchasing a built 700r4. I thought about going that route with my TH400 till I seen the price of it. This is the company I am going thru to purchase my new transmission when the time come. They have several options when it comes to what HP rating you are wanting. Hope this helps.

http://www.transdepot.net/

You could also change the gear ratio out in your axle to drop your RPM's.

To do the 700r4 swap, does anything need to be changed up to make this fit possible. Thanks! and also how do you change the gear ratio?

[GMCJOE]

I am sure there is a write up on a 700r4 swap somewhere. Anyway, you will need a new cross member for it. gmcpauls.com sells one for a great price, I think $70. And a drive shaft replacement. I know a lot of people with short beds just have there's shortened. Not sure on long beds, you would have to get some to kick in about that.

What I am talking about with gear ratio, is swapping the gears out in the axle. I am running 4:11 posi in mine. I was lucky, mine came to me with it. And is the reason for changing to an overdrive, cause like you my RMP's are running high. Plus the additional benefactors.

[dliving69]

Originally Posted by GMCJOE

I am sure there is a write up on a 700r4 swap somewhere. Anyway, you will need a new cross member for it. gmcpauls.com sells one for a great price, I think $70. And a drive shaft replacement. I know a lot of people with short beds just have there's shortened. Not sure on long beds, you would have to get some to kick in about that.

What I am talking about with gear ratio, is swapping the gears out in the axle. I am running 4:11 posi in mine. I was lucky, mine came to me with it. And is the reason for changing to an overdrive, cause like you my RMP's are running high. Plus the additional benefactors.

My identification thing in the glove box says Posi-Traction...I dont know if that means that I have 4:11. Is there a easier way to figure out what I have ratio wise...without taking off the back cover...I am still learning alot of this stuff so i appreciate it all

[truckster]

Originally Posted by dliving69

My identification thing in the glove box says Posi-Traction...I dont know if that means that I have 4:11. Is there a easier way to figure out what I have ratio wise...without taking off the back cover...I am still learning alot of this stuff so i appreciate it all

Your SPID probably has a gear ratio on it. Positraction can be on any gear ratio; it simply means there's a limited-slip differential.

If you pull the back cover and rotate the ring gear you'll see a stamp on the edge that has two numbers, something like 11:41. That's the number of teeth on the pinion and ring gear. Divide the larger number by the smaller number and you have the ratio - in the case of 11:41, it's 3.73 to 1.

If you don't want to pull the cover, there are a couple of other ways to determine the ratio. On the front of the axle tube on the right side (passenger side) there is a code stamp. If you post that, someone here can tell you what the ratio is. The code can be hard to see with 40+ years of rust, though. You can also mark the yoke on the front of the differential and turn your wheel (since it's posi, you'll want to have one wheel on the ground to do this) and count the wheel revolutions that it takes to turn the yoke one complete turn. If the wheel goes around a little over four times, your ratio is 4:11; a little less than four would be 3.73, and a little over three would be 3.07.

[weever82]

Just swap out your gears and keep the transmission if it is working properly. Pop off your back cover and read the gears, if you got some low ones like a 4:11 or 3:73; you could put in higher gears and cruise at lower rpms.
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[VetteVet]

Take your pick
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[GMCJOE]

There is a stamp on the rear axle that tells you what gear ratio you have. That's if they haven't been changed by someone else. The stamp is usually off to the right side of the cover. There is a thread on here that tells you what the codes mean.

[Fitz]

Originally Posted by dliving69

I have a Th350...So i was wondering if there is anyway you can add a overdrive? or possibly make it where it doesnt run so high on the RPM's going down the highway.

Here's the ratio's for thr 700R4, TH350 and TH400.
gear 700R4 TH350 TH400
1 3.06:1 2.25:1 2.48.1
2 1.62:1 1.52:1 1.48:1
3 1.00:1 1.00:1 1.00:1
4 0.70:1 n/a n/a

When I built my truck I looked hard at the 700R4 but that case doesn't allow the use of a robust sprag clutch pack whereas the TH400 does, making the choice of having my TH400 built and adding a gearvendors unit logical as it can take the torque of a big block.......so If you are running over 600 ft-lbs, use the "high priced' gearvendors OD unit and a built 400 with a really short driveshaft (mine came out to about 38 inches)....but, if you're running less than 600 ft lbs of torque the 700R4 will not put as big a dent in your wallet.

[dliving69]

Whats everyones thought on doing a swap to a 200r4 trans...? My friend swapped on into his and says he loves it..Better gas milage not so high on rpm..

[GMCJOE]

I am not sure if there is a difference between the ratio's between a 200r4 and 700r4. I know the 200r4 was built a little stronger to hold up to the turbo'ed engines of the Grand Nationals. It also has a universal GM bolt pattern on it. So, if you was to decide to run a Pontiac, Buick, Oldsmobile engine later down the road, you could.

Don't let HP scare you away from these overdrive transmissions. The company I am going thru offers 700+ hp rated ones. What you want to pay is what limits you. I am going with the stage 1 and gives me room to build engine a bit more.

If you can find a good transmission shop, you can tell them what you are looking for and they will build it to hold up to you. Go with what you feel will hold up to your style of driving. That's what will make you happy in the end and hopefully no regrets.