rear end swap for different gear ratio

[Mattys1970C-10]

I just got a 1970 c-10 and it has a 3 speed th400. im pretty sure the rear end is stock and has 3.08's and no posi as it tries to rip the tire off the right wheel. id just buy a ring and pinion but i dont know how to install, so im trying to figure out an easy rear end swap from a different truck. suggestions on gear ratio for a mild build street truck rarely on the freeway, and what truck do i pull the rear end from?

[TwoFiftyShifter]

Can't go wrong with a 3.73 ratio.

[Mr Handy]

I've been doing a lot of research on rear gearing, and this is what my plan is.
I have a th350 and am building my 350 longblock that will most likely never make much over 300hp.
My plan is to do a 700r4 swap at some point.
I have a 3.08 posi rear end.
I believe I can swap in a set of 3.42 gears into my rear and be just about perfect for the 700r. My understanding is that the vast majority of square body trucks with 700r trans came with 3.42 gears.
If you ever plan to replace the trans this is something to think about. Then again I live in FL and rolling down the interstate at much over 65 in my current setup is not much fun, but I have a hogepoge and tired 283/th350 combo. With the state talking about bumping the speed limit up to 75mph I do believe an OD trans is in my future

3.73 gears are popular but I think it is too low of a gear to be practical especially behind a non OD trans if you spend any time on the slab.

[Mattys1970C-10]

is there any trucks that came stock with a 3.42 and posi? this way I could just swap out the whole rear end and avoid shop labor for a ring and pinion install

[Ironangel]

I'm in the same boat you are...350 turbo trans instead of the 400...I have a stock floater 3:08 12-bolt that I'm going to convert to a 3:73 Posi, myself. I've been doing much research on the conversion and already have the magnetic based dial indicator and 20 ton press for the pinon and axle bearings...Your looking at $400 for the Eaton posi unit, roughly $300 for the ring & pinon and another $150 for the bearing & shim kit. Your going to have a grand wrapped up in parts and oils...I'm going this route for a couple of reasons. One, its factory correct. Two, I dont want to jack with welding perches and trailing arm mounts, panhard bar connections, etc. etc. Heres a link with some insight. Maybe someone can chime in with more links and helpful info we can both benefit from? http://67-72chevytrucks.com/vboard/s...d.php?t=627940

[boostedc10]

If i remember correctly the only thing close to that gear ratio in a stock 67-72 was the big block 400 trucks that had a 3:54. Most of the manuals base trucks were 3:73.

The next gen trucks had quite a few models with the 3:42 in the 10 and 12 bolts but you would have to due some welding and posi.

[Mattys1970C-10]

well wleding perches and fabbing is totally out of the question for me. i would settle for a 3.42 or 3.73 and an easy swap before i started cutting and welding. im wondering how much of a difference a 3.42 would even make going from a 3.08?

[HSRACER]

Originally Posted by Mattys1970C-10

well wleding perches and fabbing is totally out of the question for me. i would settle for a 3.42 or 3.73 and an easy swap before i started cutting and welding. im wondering how much of a difference a 3.42 would even make going from a 3.08?

A 3.42 is perfect with a 700 r trans. I have three trucks with 700 and the 3.42 is the best overall ratio. The 3.73 and the 4.11 work but are a little too high an rpm for my liking. The 3.42 is great.

[67 cst swb]

My opinion...
BBC with TH400?
If you have tall rear tires, such as something over 29" tall... I'd go 3.73
If you have shorter rear tires, under 29" tall... I'd go 3.42

BBC with TH350
with tall rear tires, such as something over 30" tall... I'd go 3.73
with shorter rear tires, under 30" tall... I'd go 3.42

Small Block with TH400?
With tall rear tires, such as something over 29" tall... I'd go 3.73
If you have shorter rear tires, under 29" tall... I'd go 3.42

Small Block with TH350
with tall rear tires, such as something over 30" tall... I'd go 3.73
with shorter rear tires, under 30" tall... I'd go 3.42

BUT, it really depends on numerous factors.
The more built the motor is, such as the more RPM the motor will turn.
The more likely that I would go with 3.73 rear gear.

3.42 is a great cruising gear
3.73 is a great performance gear
unless its a 1/4 mile only performance machine or you wanna drive it like you stole it all the time, then hell... go for the 4.11's

oh, and...
Anything with TH700R4
With tall rear tires, such as something over 29" tall... I'd go 3.42
If you have shorter rear tires, under 29" tall... I'd go 3.08

[cdowns]

going to a 342 from a 308 will increase your rpm's at highway speed and decrease your gas milage

[airdale94]

3.73 X .75 overdrive = 2.73 final drive ratio.

[cdowns]

Originally Posted by airdale94

3.73 X .75 overdrive = 2.73 final drive ratio.

yeh but 373 x 1.00 final drive ratio of a th400 equals a final driver ratio of 373

[Tx Firefighter]

Maybe it's because I drive my trucks a lot but I just can't see wanting a 3.73 without overdrive. Now mind you, I'm talking an hour commute at highway speeds but listening to that engine spinning the snot out of itself sucks. Holding a small block above 3k rpm for extended periods of time just doesn't make sense to me.

Every half ton I've built in the last 15 years has had a small block, th350 and 3.08 rearend. A stock 350 small block will roast the tires in two gears, even with 3.08 gears. And they just loaf along at highway speeds.

[scrapmetalart]

Originally Posted by Tx Firefighter

Every half ton I've built in the last 15 years has had a small block, th350 and 3.08 rearend. A stock 350 small block will roast the tires in two gears, even with 3.08 gears. And they just loaf along at highway speeds.

what kind of gas mileage do you experience?

[Cercious]

Originally Posted by scrapmetalart

what kind of gas mileage do you experience?

Been keeping an eye on this thread since I'm wanting posi in my '71. I have a 350 with a 750 CFM Quadrajet carb, 3 speed on the column, and 3.08 final. While I was in the Army, on post I got around 8 to 12 MPG. I have a bit of a problem taking my time getting to the speed limit. On the highway when I was on leave going home and back, I was surprised to find I was averaging 25 MPG. I don't have a tach, so I can't tell you how wound up it was, but my average speed between home and post was around 75 MPH.

Hope this helps,

-Patrick

[Tx Firefighter]

Originally Posted by scrapmetalart

what kind of gas mileage do you experience?

Average 12-13 MPG. I've built trucks with 3.73 and 700r4 overdrives and never got over 14 with a very mild small block. Hence the reason I just save my money and use turbo 350s and those old 3.08 rearends that no one wants. I think without going to EFI, around 14 is about all one of these old bricks is gonna get.

[oldcane]

I've gotten 14-16 on every 1/2 ton truck I've had since high school. All but this '69 have had OD. This one has a th350 and a 3.73. I have no idea my mileage since I don't have the new speedo cable yet but I'm guessing around 10mpg. It is my truck, not a show car, but I only need a truck on the weekends so it's not as bad as it sounds. But we all know these GM gas gauges. I had forgotten how they work as I've had d3500s and f250s for a good number of years now. You fill up, drive 20 minutes and it's down to half a tank. Funny how that bottom half lasts for 3 more hours. Well, 2 hours anyway.

I would definately either go to a 3.08 or a 700 trans if it was a daily driver.

Sine you guys are all over the 3.42, which is a great ratio, I've heard that the 81 trans-am (disc) rear is a 3.42 posi that will fit these trucks. Is that right? Thats 4 birds with one stone (5-lug/disc/posi/3.42). Sounds like a win/win/win/win if it's true.

[Tx Firefighter]

Originally Posted by oldcane

Sine you guys are all over the 3.42, which is a great ratio, I've heard that the 81 trans-am (disc) rear is a 3.42 posi that will fit these trucks. Is that right? Thats 4 birds with one stone (5-lug/disc/posi/3.42). Sounds like a win/win/win/win if it's true.

.....different bolt pattern than trucks use. Mismatched front and rear patterns if you go that route.

an 88-98 truck was very, very, very common with 3.42 gears (like 80 percent of them). Find one with the G80 code on the glove box sticker and you've got a locker too. And it uses the common 5x5 pickup bolt pattern.

[oldcane]

Wouldn't a 5-lug front swap be "car" bolt pattern too? What is it, 5x4.75?

sounds like a 88-98 would be handy for a number of things for our trucks.

[Tx Firefighter]

Pickup five lug pattern is 5x5. Cars use 5x4.75

[boostedc10]

You can do the 5 lug front swap in either 5x4.75 or 5x5

[oldcane]

That's what I'm saying. It would only be mis-matched if you mis-match it.

[Tx Firefighter]

Exactly what method would you plan to use to get the 4.75 pattern ?

Every 2wd GM truck made from 1971-1998 used 5x5 pattern. I'm speaking of common bolt pattern parts that can be bought at any parts store in America.

4.75 on a pickup is getting out of the realm of mainstream parts.

[oldcane]

Well, I am anything but mainstream.

http://www.mcgaughys-suspension.com/...ys-part-63150/

[boostedc10]

Originally Posted by Tx Firefighter

Exactly what method would you plan to use to get the 4.75 pattern ?

Every 2wd GM truck made from 1971-1998 used 5x5 pattern. I'm speaking of common bolt pattern parts that can be bought at any parts store in America.

4.75 on a pickup is getting out of the realm of mainstream parts.

He was asking about a T/A rear axle which is the 5x4.75, so if he did swap it in he still could get a front conversion in the same bolt pattern.

I don't think anyone said our trucks were 5x4.75.

5x4.75 is just as mainstream as 5x5 (which is why they offer both patterns in the brake kits) just not for the full size.