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Gears steers an qu... well just gears

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Mudslutk51984

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My current set up is 35/12.5 15's on 3.73 rear open and 2.73 front... I didn't do it!!!
I bought it that way... so since I'm going to have to do a gear change to use the 4x4 I was curious what fellow k5er's have to say about matching 3.73 ratios and 35's... I do a lot of off road mud hills no rock crawling, and occasionally drive to town but never freeway.. mostly 55 mph zones. I'm pretty happy with the 3.73 rear but is there a better world that I can't see?!?! WooooOOOooo
Thanks guys
 
oh if you have 700r4 and over drive with 35" tires go deep .

the over drive helps a TON with rpm at highway speed.

lots of guys here do min of 4.56 with this size tire/tranny combo so the trans dosnt hunt 3-4 gear at speed.

current setup 3.73/35" tire in overdrive at 60mph is 1500 rpm
4.56 swap would get you 1840 rpm at 60mph

and 5.13 gear swap most have done and like will be 2070rpm at 60 mph .

sbc tend to like the 2000-3000 rpm band for good power band . so this wouldn't be a bad thing .

might be worth looking for a good pair of used axles to swap in with better gears already over the price of regearing yours in the end.
 
then its not a 700r4 tranny . its a th350 or th400 . or its ready to die

mph 60 and rpm

3.73 = 2270
4.56 = 2775
5.13 = 3120

the th350 or th400 is 1.00 in 3rd and non lockup unless rare lockup th350 model .
 
sweetk30 said:
then its not a 700r4 tranny . its a th350 or th400 . or its ready to die

mph 60 and rpm

3.73 = 2270
4.56 = 2775
5.13 = 3120

the th350 or th400 is 1.00 in 3rd and non lockup unless rare lockup th350 model .
Click to expand...

Are there no locking TH400 cases? I was thinking there were a few exceptions.
 
THM350C (lock up convertor) is quite common and IIRC they were standard in all GM vehicles that used a 3 speed RWD auto from about 1966-67 on.

In all my years as owner/operator of a transmission shop, I've never seen a THM400C, or a 400 lockup transmission. There may be some out there that never came in my doors, but I doubt it.

There were a few THM200C units, and of course the THM125C and THM325Cs, the latter two being front wheel drives. The "C" designation always used to indicate a lock up convertor.

ALL GM 4 speed automatic have Overdrive built into them as "4th gear" which confuses people who think that the bump they feel when the converter hooks up, is Overdrive, when it's not so.

To test if it's the converter or a true gear shift, just rest your foot on the BOO Switch, just enough to light up the brake lights but not actually apply the brakes, and the lockup clutch will release with some RPM increase in the engine speed.

The advent of a locking converter improved fuel mileage, emissions control and reduction and helped cool the transmission from heat buildup from a torque converter whipping ATF into pink foam.

If someone installed an auxiliary overdrive like a Gear Vendor or the like, then there indeed is a 2-speed over/under, over/over or under/under on the tail housing of the transmission..... but having one on a 4WD transfer case is pretty near unlikely. Not impossible, but very unlikely!
 
Bowtie85 said:
My 80 k10 had a rare t350c.......it's been chucked in the bush :haha:
Click to expand...
Actually, that other post about a 350C being a tad weaker than the THM350C is BS.

And.... the THM700R4 had a lot of the same parts in it from the THM350C. The 700 is just a next generation 350 with some other parts thrown in and a higher numerical ratio Low Gear (1) gear with extra pinions in the truck versions and a few extra steels and frictions.

Weaknesses in the THM350C and the THM700R4/4L60E just don't hold water until you get above 300 horsepower or you get an idiot driving it and manually shifting it incorrectly. Any idiot can destroy anything. ...including a cast iron ingot.

I have had HD sprags grenade on first shift right off the hoist though, so I guess I qualify as an idiot too.

For the record, the THM350C was actually made in greater quantity than any other GM transmission according to ATP and RPM companies but that was said in 2003 and may be subject to revision.

The THM700R4 and it's latter hybridization versions are by now likely the #1 transmission in the world in units produced

Back to the THM350C.....it was however rarely installed in a G body unless it was a G body station wagon. RARE BIRDS!

S10s and some vans had the THM350C most of the time unless some unfortunate dolt didn't see that a THM200C or a THM180C was on the order form as OE when they bought it.

Not including the light duty THM180C, all THM250C family units used the same pan with 13 pan bolts and are easy to screw up identification to the novice "rebuilder".

For quick identification of the less desireable 250C, check the passenger side of the case, a 250C will have a band adjustement (looks like a stud with a locknut). No other transmissions in this family had an adjustable band.

There was a THM375B factory built as a heavy-duty version of the 350 and can not be distinguished without disassembly or checking the transmission code.
 
I have a TH350C tranny in my '81 G-10 van,that came factory with the 250 straight six it had (till I put a 307 V8 in it,coupled to that original trans)...it had 180K on it when I parked it back in '04,but was still working well then..now might be a different story,the long nap has not done it any justice...

I also have a identical spare tranny from an '86 G-10 I parted out for the engine for another 2wd truck I had--seeing it was working fine,and would have fit that truck and my van,I kept it --they had the funky vacuum switch on the firewall to operate the converter lock up function..

Far as I recall GM didn't start using lock up converters until the late 70's when everything was "downsized" for better fuel economy.
I don't recall seeing a TH400 with a lock up converter--there were "switch pitch" torque converters in the early TH400's in some GM cars from '64-66 though,both of my '66 Buicks had them..
 
diesel4me said:
I have a TH350C tranny in my '81 G-10 van,that came factory with the 250 straight six it had (till I put a 307 V8 in it,coupled to that original trans)...it had 180K on it when I parked it back in '04,but was still working well then..now might be a different story,the long nap has not done it any justice...

I also have a identical spare tranny from an '86 G-10 I parted out for the engine for another 2wd truck I had--seeing it was working fine,and would have fit that truck and my van,I kept it --they had the funky vacuum switch on the firewall to operate the converter lock up function..

Far as I recall GM didn't start using lock up converters until the late 70's when everything was "downsized" for better fuel economy.
I don't recall seeing a TH400 with a lock up converter--there were "switch pitch" torque converters in the early TH400's in some GM cars from '64-66 though,both of my '66 Buicks had them..
Click to expand...


That vacuum switch on the firewall was to disconnect the LU if you needed to accelerate and get the converter into 'torque conversion' mode so the engine could get some RPM and pull a little harder. It's supposed to read pure manifold vacuum, but I've seen people make a big mistake and attach it a modulated vacuum source and really screw up the LU pattern.

IIRC.... that same switch was used --- either here or on another vehicle --- as an AC compressor dropout switch for high throttle demand during passing conditions.

I still have that switch on the firewall of my '86 Blazer. I might find a use for it someday..... who knows.

I did some experimenting with my LU by wiring in what I call a "burp switch" pressure sensor from a THM200R4 on the valve body that reads the TP and kicks out LU while still letting LU and shift progressions happen normally. I still use the BOO Switch to disconnect LU when the brakes are applied.

The transmission is a mash-up of several things to get it to do what I needed in my working toy. I love the Kevlar 2-4 band I had made. With the Corvette servo, it's a really clean, firm shift.

I also drilled out the LU apply circuits to get it to apply harder. In over 100,000 miles, no problems yet. I also added a switch alongside my console to turn on or off the lockup circuit if I get too much shuttle-shifting busy-ness when I'm pulling a load.

The best parts buy i made was a set of (military??) straight cut transfer case gears--- noise it makes--- but power and reliability is super enhanced! Bless the US Army Field Design and Purchasing agents!

I really want to know more about that THM400 special convertor. Hmmmmm! Google is my friend.... but later tonight for that as my thumb's getting tired on my tablet at the breakfast table.
 
Back to the OP, the cheapest and easiest solution would be to source a complete 3.73 geared front axle from the junkyard. Assuming the front is a 10-bolt or a Dana 44 you should be able to find one and should be cheap......say that because I haven't looked at those parts in years and should be getting more scarce, but many years ago you could typically buy a complete 10-bolt or D44 front axle for $100-$150. Unbolt the current axle and bolt in the new one, or you could always swap the gears and carrier around or otherwise mix and match. Labor is pretty straight forward but the parts (axle) is big and heavy, and rusted hardware can cause issues. On a hoist and with power tools I've completely removed a front axle from mine, drilled out broken bolts, bolted it back together and drove it out the door in a couple of hours.

Next option is to install new 3.73 gears in the front. Probably at least a couple hundred $$ for the ring and pinion and parts and then labor. There is more labor, especially if you haven't done gears before, and requires some special tools.....definitely not a unbolt the old and slap in the new stuff if done correctly. I will say gear setup on the front is not quite as critical as the back because most people only have the vehicle in 4wd for a small fraction of the time and speeds are usually low.

New front and rear gears of a lower ratio would be ideal but you are getting into a decent chunk of money.
 
Is say 4.56 if you have overdrive. If it
Is three speed 4.10 or 3.73 is ok without over revving the engine at highway speed. Will be much easier to dripnon a replacement jy axle if you can find one the correct gear. 41 11 is 3.73. The front is not quite as crucial as was said but have to get it At Least close with correct shims and dial indicator for back spacing the gears. I bought used gears for
Mine and haven't had a problem with them. Did replacemthe bearings and seals while I Was in there tough.
 
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