Rear Gears!!! Help please!

[71ChevySS(straight6)]

Hello, everybody! Been a while since I posted, but I've got a question for ya. Rear gears. I'm pretty sure I've got 4.11s, I don't have a tach, but I know my six has to rev high to keep 60 MPH, and since I just dropped a couple grand to have it rebuilt, I don't want to work it hard. I don't need the low gearing anymore, I won't be towing or hauling much with it; higher road gears will be more suitable for what I'll be doing. I'm thinking of going to 3.55s.

How difficult will it be for me to do it myself? It's a 1971 half-ton, New Process 435 4-speed. I don't have the truck with me or nearby at the moment, so I don't know exactly what rear axle I have, but maybe with the given information you guys can help me out.

Any help is appreciated. Thanks a bunch!

[67chevemall]

3:07 would be a nice gear for your set up. Nice job on the 4 speed.

It can be done at home just take your time and get ALL the tools you need
before you start.
Watch lots of Youtube videos to help picture what you are getting into.

And satin black with the "hotwheels" red stripping is a solid choice!
Red stripe wall tires would be sweet to.

[picklito]

Decide on the diameter of rear tire that you want to run, then look online for the gear/mph calculators. And yes, I'd be thinking closer to 3.07 with that 4-speed.

[GASoline71]

What is "high" RPM's to you... is actually pretty normal for these old trucks. If you're worried about "wear and tear", no need... they will run at 3,500 RPM's all day long at highway speeds. Even after a fresh rebuild... the SBC's will be just fine at those RPM's.

Modern fuel injection and awesome overdrive transmissions have spoiled us.

Gary

[Redline novdog]

Decide on the diameter of rear tire that you want to run, then look online for the gear/mph calculators. And yes, I'd be thinking closer to 3.07 with that 4-speed.

I'm with Picklito try a 31" or 33" tire before changing gears. A whole lot cheaper and if you do the math you can decide on a gear that suits you. I have had 33,31,and now have 30" on the back with a 3:08 rear and 3spd auto. No tach sounds/feels good.

[Stocker]

This might help -- http://www.huntergraham.com/twcc.htm

[mike16]

to go from a 4.11 or a 3.73 to a 3.08 rear you will need more than a ring and pinion. you will have to locate that part the ring gear bolts to. dont know what its call off hand but i looked into it and you'll need that part. another benifit is improved gas milage. my 72 witha 307 2 bbl and 3.73 gear gets about 13 mpg at 55 mph. a frind has a 350 4bb with a turbo 350 tranny and 308 gears. he gets 17 plus mpg.

better to have a pro do it or find a complete rear with the 308 ratio. Pic a part in las vegas off of bolder sells rear ends for 110 bucks drum to drum.

[Mike C]

I've done a bunch and it is a skill that can be mastered and completed with some special tools. Realistically, to do it right you will need many hundreds of dollars worth of tools. But I would suggest if you haven't put the effort into doing research and just asking the question I don't think I'd start there.

3.73 would be the stock gear, but since you have the 435 which is not that common an option, I would check the SPID to see if there is an optional gear ratio as well.

The easiest, and cheapest, way to get different gears is to buy a complete rear axle from another truck that has the gear ratio that you want already in it. Then just swap axle housings.

Another way to get the gear ratio down is taller tires. I had 31" on my short bed and it put the speedometer off by 10 miles per hour on the highway so figure you can get a pseudo overdrive that way. Now that the truck is lowered I went to 29" tires and it's only off about 5 mph.

[Shaky]

I started with a 3.73 open rear. I put an Eaton cone-clutch posi and 3.07 gears in. All new bearings.

The most difficult part is setting pinion depth.

If you are going to do this for the first time, I suggest going slowly. I did it over the winter, in the basement where it was nice and warm and well lit.

[special-K]

3.08s is what these trucks use. 3.07 is a car ratio. What is the diameter of your tires now? This is all guess worth without knowing tire diameter, axle ratio, or what RPM you are running at 60.
It is true these trucks can run down the hiway as they are now just as well as when they were new. The issue is the younger drivers who don't know anything but experience with overdrive. There were interstate hiways in 67-72 and the speed limit was 70 mph most everywhere. Here in Maryland it is 5 mph slower than it was back then. The other issue is all the morons use overdrive as a way to drive faster. Screw them, they can change lanes and one requirement of driving that will never change is you are to look where you are going. The purpose of overdrive is to reduce wear and tear and reduce fuel consumption, but driving faster negates that. It's a good idea but not a requiement...and neither is the speed "limit".
I was driving my '95 yesterday wondering why overdrive automatics will jump to overdrive when you let off the gas at 40 mph. I mean really? Should be more like 50. That's how I drive a manual overdrive. But manuals require you to think. No wonder people are driving 50 mph in a 30 zone and tearing through parking lots like they are on the road. I usually take it out of overdrive in lower speed zones. You don't want overdrive running through town or neigghborhoods. It seems so many people are either on their gas or brakes and don't realize you can lift your foot and the car will continue to maintain speed for long distances, and if you are going up hill the car will slow down when you lift off the pedal. Ok, so I went into a rant

[Mike C]

That cone clutch posi is an Auburn and not at Eaton. I would never discourage someone from doing the work and learning a skill. I will buy a $1000 worth of tools to save $250. And have NEVER regretted it.

But when someone can't even take the time to figure out the gear ratio in the truck by using Google to come up with ways to answer that question, I figure they won't put much effort in learning to set up a differential which is not just guess work and shouldn't be taken on if not going to go all the way. Like working on it over the winter with plenty of time.

And SpecialK is right on as usual with his analysis.

So for someone new without a whole bunch of interest in learning a technical skill that will require a significant investment in tools in order to do correctly, the best option is to determine what you have and what you want, and buy a used complete setup that fits that goal.

[71ChevySS(straight6)]

Originally Posted by Mike C

But when someone can't even take the time to figure out the gear ratio in the truck by using Google to come up with ways to answer that question, I figure they won't put much effort in learning to set up a differential which is not just guess work and shouldn't be taken on if not going to go all the way. Like working on it over the winter with plenty of time.

And SpecialK is right on as usual with his analysis.

So for someone new without a whole bunch of interest in learning a technical skill that will require a significant investment in tools in order to do correctly, the best option is to determine what you have and what you want, and buy a used complete setup that fits that goal.

Okay. First of all, I'm going to college this fall, at the rate of $33,460 A YEAR, to achieve my Bachelor's in Automotive Restoration (that's four years, times $33,460 per year. Luckily I have some amazing scholarships and only owe a little over $6,000 for freshman year at the current moment). There's one reason I don't want to invest much in this project.

The other reason, and the same reason I don't have all winter to work on it: I have to drive this truck to college. I can't be stuck out there for months without a vehicle while I tear apart and gradually redo the rearend.

I may go out and find a good rearend with 3.55s, since it's apparently the quicker, cheaper way to go about it.

You really shouldn't assume stuff, Mike, but thanks for the help.

[picklito]

Originally Posted by special-K

Ok, so I went into a rant

Ya but you're right!

[RichardJ]

I think the 3.55 you mentioned would be a better choice than 3.08 for a stick with small engine.
Automatics can generally handle taller gears and that is what some of these guys have in mind with that recommendation.
The chart below is from an Off-road web site so you can take it for just a comparison reference only.
They put 3.07 gears at the bottom of the useful range and even then only with a 28" tire which is a pretty small truck tire.

Personally I think even for KS, your 250 would struggle with hills, head winds and much of a load with 3.07 gears.

I swapped the 4.56 gears for 3.54 gears in my truck. The most noticeable bang-for-the-buck on my truck next to AC. 75 on the hwy and can handle most hills without straining.

[Mike C]

Good info for him Richard, but not worthwhile if he has 3.73 already, just too small a move % wise.

Really need to know what ratio is in the truck now and what size tires he is running and wants to run to be able to really make a good recommendation.

And 3.31-3.55 would be a great street ratio, especially with the NP435 trans versus a 3 speed.

[71ChevySS(straight6)]

Originally Posted by Mike C

Really need to know what ratio is in the truck now and what size tires he is running and wants to run to be able to really make a good recommendation.

I'm sorry, I don't know that at the moment, my truck is 20 miles away getting the rebuilt engine reinstalled. And I don't have the time to just skip over there and check on that stuff. But I'll let you know as soon as I get the truck back.

[mike16]

Originally Posted by Mike C

Good info for him Richard, but not worthwhile if he has 3.73 already, just too small a move % wise.

Really need to know what ratio is in the truck now and what size tires he is running and wants to run to be able to really make a good recommendation.

And 3.31-3.55 would be a great street ratio, especially with the NP435 trans versus a 3 speed.

I agree, its not really worth the effort to only go from a 373 to a 355. you do that cheaper with taller tires.

If your truck is done hauling in its life the 308 is a good leap forward with the added bennies of better gas milage. I would do it to mine except I still work it. but at the time I was considering the change over it was a daily driver and at 70 miles a day the bennifits of mpg made sense. its not cheap unless you find a good rear end in the pic a part.

[71ChevySS(straight6)]

Originally Posted by RichardJ

I think the 3.55 you mentioned would be a better choice than 3.08 for a stick with small engine.
Automatics can generally handle taller gears and that is what some of these guys have in mind with that recommendation.
The chart below is from an Off-road web site so you can take it for just a comparison reference only.
They put 3.07 gears at the bottom of the useful range and even then only with a 28" tire which is a pretty small truck tire.

Personally I think even for KS, your 250 would struggle with hills, head winds and much of a load with 3.07 gears.

I swapped the 4.56 gears for 3.54 gears in my truck. The most noticeable bang-for-the-buck on my truck next to AC. 75 on the hwy and can handle most hills without straining.

Thanks for your help!

[Mike C]

I'm not trying to be mean and would like to help, but you didn't offer a lot of information up front and asked things that can be answered with Google or searching on the board. And can't be answered without the truck. (Which you did say upfront I might add!)

I learned all about rear axles with my own truck because I kept breaking them. And it was my transportation to and from college as well. I didn't have the luxury of having it down for long periods of time. But there were several instances that took extra time and required bumming rides from Austin to Lubbock and back. I was fortunate that the worst failure (broken timing chain at 252k miles) happened 5 miles from home.

I will still add that in your case the best choice will be to buy a complete axle with appropriate gears. The NP435 should be synchronized in all forward gears and 3.07 will work well I think. My '79 CJ5 has 3.54 gears and the 2.97 geared 3 speed and 33" tall tires and it does on OK job. You will have more weight but shorter tires I'm sure. And the good news is that 71-72 axles with 3.07 gears are not hard to find. If memory serves, the first gear in your trans is in the mid 4's.

[special-K]

I agree on 3.08s being tall for a (virtual) three speed...or manual at all. I personally draw the line at 3.42s even with a car 4spd or anything, auto or manual, with overdrive.

The best thing I ever did was buy "set-up" bearings for the 12-bolt. I still don't have a press, but I can put together/take apart as much as needed to get the right pattern, then go have the permanent bearings pressed on. I could probably manage them on by primitive means but never tried because I get real anal when doing tasks where precision is paramount. It's a quick & cheap task at a shop or not much to ask of a buddy who has one.

With the tools, a proper manual for specs/procedure, and diligence/care anyone can do this. Without any one of those things (or good luck) you can easily spend far more than paying for the job. A straight rear swap is the simplest route, as well as cheap

[19673ontree]

My truck is bone stock 6 cyl 250 with 3spd on the column mine came with a 3.73 I put a 3.08 in it and made the truck a lot better for long trips , I got my 3.08 out of an 80's 12 bolt truck rear end it fit and worked perfect ! I did mine at home as I do everything you can do it at home , the biggest mistake ive seen with people who install gears is the set them up too tight when done you need to be able to feel some slack in the ring gear by hand and use grease to check ur pattern , good luck

[djgebert]

When I first bought my truck I drove it for about six months before I retired it and turned it into a toy. It had 3:73's in it when I bought it and I switched it out to 3:08's to make it more pleasant to drive. IMHO you will be very happy with the 3:08's as a daily driver, they are almost a perfect all around gear for daily driving, especially if you will be on the highway at all. With 3:55's you will most likely hardly notice a difference if you have 3:73's now. That 3:08 swap made my truck go from a vehicle that was barely driveable on the highway to a well mannered driver overnight. The 3:08's have plenty of low end for any normal use, heck I could still smoke the tires at will with a mild 350. Of course now that is all out the window and I've went completely the other way, lol, but my truck is now a retired toy. So my vote would be 3:08's, and you should be able to find a whole rear end with 3:08's dirt cheap and just swap it out.

[67ChevyRedneck]

You're not going to notice much difference at all going from a 3.73 to 3.55, maybe a few hundred rpm, which is kinda "nothing"... and what came with a 3.55? That's not a common ratio I remember being in these trucks...

The vast majority of manual trucks came with 3.73's. The vast majority of automatics came with 3.08's. The cheapest, quickest thing for you to do is to find a 3.08 open rear (they're not as "popular" as the 3.73 or posi), look around the pumpkin for signs of heavy leaking (leaks around the rear cover are OK, a new gasket can fix that, but a leak at the yoke requires a lot more work) pop the cover, inspect it for wear, pull the drums and look to see if the axle seals are leaking, and if all looks good, swapping the rear axle can be done in just a few hours and maybe add a day if you have to redo all of the brakes. Also remember, 67-70 rears will be 6 lug and 71/72 will be 5 lug. It looks like you have a 71, so you'll need a 71/72 5 lug which is more popular because of the 5 lug pattern... realistically, expect to pay $150-250 for a good useable rear. I picked up a 3.73 5 lug rear last year that with a little clean up could be run under my truck immediately for $250. These trucks have really well built/tough rear axles. I'm pretty sure mine has easily a couple hundred thousand on it and it still has the original axle seals, the pinion doesn't leak, and when I replaced the oil a few years ago the gears looked new inside still, so a good used rear should be easy to find.

Your truck will be fine with a 3.08... hundreds of thousands of them came with the 3.08. AND most people that have swapped to OD have 3.73 gears... which honestly, by the time you do the math... having OD with a 3.73 is almost equivalent to having a 3.08 rear and no overdrive...

[Jason Banks]

I'm in the same boat. 250 6cyl 3 OTT. Factory 4.11 rear gear. According to the calculator linked above I have 28.88 inch tall tires.

It feels pretty comfortable cruising at 65 and even 70. I don't have a tach but I would say the chart above is pretty close 3095 RPM for a 29 inch tire and 4.11 gears at 65 MPH.

I have a GPS Speedometer on my phone that I use occasionally to check my speed and I'm pretty sure I have never hit 75 mph. It just seems to be revved pretty high.

[rharp]

OP, if you're interested in just buying a different rear end to speed up the process, I have one that I had a prospective buyer, but he may be going a different route. The only catch is that it's a '69 rear end, so its going to be 1.5" narrower than your current one. It will bolt up just like your other one, but if you're running max width wheels/tires, it may ruin that for you. It's 3.08 gear.