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1949 Chevrolet 3100 pickup with stock 4:11 rear end. Thinking about changing rear end to 3:55 for highway driving. Kits are available to do this for about $1,000 using existing rear axle and drive shaft. However, would it be better just to get a used rear end from a salvage yard that meets the needed spec and just bolt it up to the drive shaft. If this is possible what are some rear ends that would work? Thanks

You have an enclosed driveshaft?

The likelihood of finding a 3100 enclosed driveshaft with 3.55 R&P in a salvage yard is very unlikely.

An open driveshaft and 3.55 rear-end would require a transmission change and a correct length driveshaft.

If he has a sm420 trans, for an open change-over, he would just need to remove the rear yoke assembly off the trans, get a 55 or later open style rear bearing retainer and install the seal and find an open input shaft, no removal needed.

First off, the difference between 4.11 and 3.55 is only about 14%. This isn't a great reduction, and going to an open driveshaft/different differential will offer you a great many better ratios. You can get as far down as 3.07 with 6 lug wheels and a 12 bolt rear end or down into the 2s with 5 bolt hubs. However, there are some things which are often overlooked (but which will in the process become concerns you have to address). Please let me point out a few of them:

1. You will have to buy a different driveshaft. Depending on the transmission you choose, you may have to measure and have one custom made or find one in a junkyard which happens to be the right length...and this can be a real needle in a haystack with the way junkyards are scrapping metal today. If you do find one, the trans yoke you use most likely won't be balanced to it. Count on a new driveshaft (balanced with new U-joints and a matched yoke shaft) costing around $400 to $450 delivered to you.

2. Your emergency / parking brakes won't be the same any longer. You will need to fabricate a new or unique cable setup so they will work now.

3. Unless you're fortunate enough to find a rear end with 6 lug wheels, you will need to find new front hubs (with 5 lugs) plus new 5 lug brake drums or carry two spare tires...one 5 lug and one 6 lug.

4. Your new differential won't mount correctly to your stock leaf springs without some work and even then it isn't likely to be ideal. The center pin in your rear springs isn't in the right place because in your closed driveshaft system, your stock differential mounts completely differently to your springs. The spring mounts provide the rotation needed due to the missing rear U-joint in a closed driveshaft system. In addition to this, even if you find a differential with leaf spring mounts on it and even if those mounts match the inward bound spring angle in your pickup (and I haven't seen one yet), you will have to remove those spring mounts and re-weld them after your pinion angle is established or figure out the pinion angle and shim those existing spring mounts in much the same way the front end is shimmed. If your pinion angle is wrong, you will wear out U-joints rapidly, you'll wear out your trans tail shaft bearing and your pinion bearing and listen to it howl and growl while that is happening. Wearing out U-joints rapidly can be very dangerous. If you've ever seen a vehicle lose a driveshaft at highway speed, you know how ugly it is. The sound of a bad pinion angle is sort of like whumm, whumm, whumm in a car with carpet and insulation and in a truck it is like driving next to a very old cement mixer truck.

5. Your old brake hook-up (the flexible line) probably won't match the new rear end and you will have to do some brake line work there. It is very probable you'll have to relocate where the flex line connects and this means welding on a different junction mount tab.

6. If you are changing the rear end, you may as well change the transmission so as to get a newer and better designed one (with at least all forward gears synchronized) and if gas mileage is your goal, you may want to go with a T5 transmission (which will either require finding a Jeep front shaft or making up a new clutch/pressure plate/throwout bearing combination). In this case, finding an 11 inch flywheel will be by far your best option and you may have to search for one of these. If you don't use the 11 inch flywheel, your options are to use a very small (8 7/8 inch) pressure plate/clutch combo to find one with a matching spline or have a rebuilder make a custom one for you if he can find the right hub. Clutch and pressure plate rebuilders have told me they think the 8 7/8 inch stuff is to be avoided on anything with more oomph than a 4 cylinder. One man in Waco told me he won't even rebuild them any longer. If you go with a T5, you must find one with a mechanical speedo output (and change the gears to match your situation) or use an electronic speedo. Or you might find a 3 speed with overdrive. Find one with a synchronized first gear if you go looking.

7. Whatever you do, save your old shock absorber mount plates. You'll need them because your "new" differential's shock mount system likely won't match your old one. You will also need new U bolts and they'll need to be really good ones.

Good luck. Changes like this are not that bad if you can do most of the work yourself and they make things better but there will be what we used to call nickle & dime expenses. Today they're $100 & $200 expenses, however.
Jon

All before is correct. But I installed a 70 chevy nova axle in my 54 3100 the problem there is the width of the spring pack, wider. I also made a lowering block out of square tubing and adjusted the axle to center the axle in the fender opening. I wish I could be more articulate about this but it has been 20 years. I may have just lucked out, I went to the junkyard and found the right length but had to change the u-joint. I had also lucked into a transmission that was a direct swap. Good Luck.

Depending on which engine he is running, he may not want to go more than 3.55. We just replaced my rear end and went from 4.11 to 3.55 so I could run highway speeds. At 3.55, my 216 struggles on starting up hills and I lost a lot of torque. However, I can go 70 mph on the interstate at less than 3k rpms.

We need to know which engine to assist him correctly. I would hate for him to go to 3.07 if his engine doesn’t have the power to move the truck. His tire size will also play s roll in this.

Bucktruck, if you change your rear gears, you should also do your axle seals and all. It will probably cost you about 1,000 to 1200. I recommend finding someone who has already done this and is moving to an open drive line. That is how I got mine for $600, they helped install, trustworthy Stovebolt member who had 500 miles on it, and no issues.

Just my $0.02,

Chris

235 engine

Buck, here is some info in case you're still thinking about this.

Here's what you have as your final ratios currently (with stock 4.11 rear)
1st gear is 2.94 x 4.11=12.08
2nd gear is 1.68 x 4.11=6.90
3rd gear is 1.00 x 4.11=4.11
At 60 mph and 28 inch tires, your engine should be turning ~3000 rpm. At least mine always did.

And, if you go with 3.55 and keep your stock transmission, here is what you have:
1st gear is 2.94 x 3.55=10.437
2nd gear is 1.68 x 3.55=5.96
3rd gear is 1.00 x 3.55=3.55
60 mph and 28 inch tires should have you driving at 60 mph and turning ~2500 rpm. And I should mention you'll stay in 1st gear for about 26 feet (as opposed to the stock setup which will have you shifting out of first at around 16.5 feet). Obviously you will spend more time in 2nd gear also.

A take-off ratio of 10.5 is acceptable if you mainly drive flat streets and are not going to be using your truck to haul heavy loads. If you plan on using your truck to carry firewood or something heavy, I'd try to get closer to the ratio the truck was designed to have. From Chris's comment, he might agree. When Patrick first had the 3.55 gears made I went out there and we drove his truck one very hot Arizona day. I should add he had a 4 speed with a 3.11 first gear and this made his take-off ratio a bit over 11. He had also lightened his truck with monoleaf springs. He and I talked about it and he said he considered using the 4 speed with the 3.50 first gear to get the higher take-off ratio but the % drop-downs in that transmission were not as favorable as in the one with the 3.11 first gear. I thought he had plenty of pickup with the take-off ratio of 11 and change.

On the flip side, if you went with a T5 with the gearing shown below and a 3.07 rear end, you would have:
1st gear is 4.03 x 3.07=12.37
2nd gear is 2.37 x 3.07=7.27
3rd gear is 1.49 x 3.07=4.57
4th gear is 1.00 x 3.07=3.07
5th gear is .86 x 3.07=2.64

These ratios were calculated using 15 inch wheels with a tire diameter of 27 inches. 28 inch tires will lower them to almost exactly the stock numbers. If you look at the first 3 gear final ratios, you'll see they are very close to the original 3 speed ratios. This means in the first 3 gears of the 5 speed, your truck won't feel greatly different from stock and if you need to haul a half ton of firewood home, you'll have plenty of gear to do it. When you get to 4th, you will be effectively in your first overdrive gear with an overall ratio of 25% less than your 4.11 rear end. That's significant relief for your over-revved 6 and you could cruise in 4th at 60 mph at about 2300 rpm if you wanted to or you can use that gear for hills or passing if needed. When you get to 5th gear, you'll be 39% lower than the 4.11 and at 60 mph, your rpm (again with 27 inch tires) will be about 1970. I don't believe you want to go below 2000 with the 235 for a cruising speed, so 1970 rpm is what I'd consider the bottom line.

Good luck!
Jon

Jon,

I agree with you. The 3.55 was my max. FYI, staying close to the fort worth stockyards tonight and tomorrow.

Buck,

Figure out your plans for the truck. are you hauling, pleasure rides, running moonshine, or driving in hills? Depending how you answer that will answer the question of how fast and which transmission/rearend. There is a lot of knowledge on here that can guide you. obviously jon is one of those. I can only speak about my truck and what i have screwed up.

Good luck!

Chris

This is a little big apple little apple comparison but my 216 engine 3600 is about perfect with 31" tires, 4.10s and a .73 5th gear T5. When things get hilly you forget 5th on the up hill side but there are enough gears to make it work. Now with a higher hp 235 it's really be a nice package. However the stock 216 when new was rated at 85 flywheel hp. Now 67 years later it's probably making more like 75 hp and rear wheel is likely 65 or so. I may have to strap it down to a dyno one day just to see how many wild horses I'm trying to control when going down the road.

ndkid275 said "FYI, staying close to the fort worth stockyards tonight and tomorrow."

The White Elephant across from the Stockyards Hotel may have openmike night, tonight (Tuesdays). Worth a visit if that's the case.

Chris, I hope heat doesn't bother you. Rumor has it we'll hit 104~105 today.

Moparguy, yes...offhand that looks very close to the same as a .86 5th gear and 3.07 rear when you add in your 31 inch tires. The first car I owned was a 1936 Ford and as I recall it had 85 horsepower. Even so I never thought it was underpowered. It was originally purchased at the State Fair of Texas and had a special shift knob commemorating 100 years of independence. I kept that knob when it was sold and replaced it with a Pabst Blue Ribbon knob (which I wish I still had because I imagine it would be nearly as uncommon).

Jon

I think I will just stick with the stock setup. I only drive for pleasure around town and never haul anything. May try a parade some day.

Start with stock (like you stated), drive it and see what you think. If you feel like you need to change in the future, do it then. Save the money, plan it out and enjoy your truck! Obviously, we need pictures of your girl.

Chris