What gears for my application?

[sierrablue]

So I have a leaking pinion seal on the rear end on the Jeep, and in the front it needs new spider gears (the rear may actually need them too--not 100% sure yet). Anyway I have to pull it all apart at some point anyway, and I know that these 3.54s are great for 55-60 (or less), and they'll go all the way up to top out the speedometer, BUT the engine could be happier about cruising at 70+. Down here at like 1500 feet it'll almost spin one of the tires on dry pavement, BUT I'm worried about high altitude. Obviously my output is gonna be reduced as I go up. I don't need this much torque down here, and I know the long trips that I'll be doing with it in the near future are going to be lots of 65-75 mph speed limits.

I already have one set of D44 3.31s--would it be worthwhile to install them? I don't want to mess with it if they're not really going to help me much on the highway, and/or they're going to seriously hurt its abilities at 10k+ ft.

I know the right answer is an overdrive. However, since I want to do an EV conversion, I'm not investing serious money to make the gas drivetrain perfect now. That's how people end up never doing conversions and getting stuck being reliant on gas forever, which is not what I want.

I suppose another set of Yukon gears is ~$215. I'll already be spending money on new spider gears, and I know the gearing itself is a pain. Then I'll need a speedometer gear I'm sure...

NOTE: I'm not doing hard-core off road stuff in this thing. I use it in the deep snow, and I'm not afraid to drive it in the dirt or anything, however it's not a trail rig, it's a daily driver. The XC mountain bike on the back is for exploring; the Jeep is to get me to the trailhead or whatever, or home, or wherever I need to go regardless of the weather. Just wanted to clarify that before everybody tells me to put 4.56s in it.

[will e]

Tire size would help.

check out Grimmjeepers gear calculator.

http://www.grimmjeeper.com/gears.html

[Stuka]

3.31's will be slightly more HWY friendly. It won't be night and day. Its actually easier to put one size larger tire on if they fit and get the same effect as far as engine RPM at speed goes.

At as you mention, at altitude, you really want lower gearing. I am actually a fan of overgearing, but I also live in the mountains. In my old JK, and in my J10 I ran 4.56's with 33" tires. The J10 did not like speeds over 60, but it was a 258 and didn't do freeway trips after that. The JK had a 6sp manual, so it had over drive, and the V6 happily turned 2600rpm on the freeway without issues. My current JK has 4.10's, and I actually want to regear it too.

Going from 3.54 to 3.31 does not seem like its worth the cost of the gears and the labor to install them to me. If those other axles are in really good condition, and you don't have to replace all the bearings and ball joints, that could be an option. Heck, you could install just the rear and drive around and see if you like it since you have a D20 t-case.

[tgreese]

... Going from 3.54 to 3.31 does not seem like its worth the cost of the gears and the labor to install them ...

[sierrablue]

235/75r15s

I know I can calculate the RPM; that doesn't actually help me decide which way to go, as I don't have a dyno graph, nor do I know what percent of power it's going to lose (since all the numbers I'm finding are for mass-air FI systems) at altitude, that doesn't do me much good.

I don't know what kind of shape the bearings are in, but I know I have to pull things apart to address the slop in the spider gears anyway. Driving it daily I haven't been keen to take it apart and figure out what all it needs. Needs a pinion seal in the back for sure.

I already have one set of the gears, and I have all the tools. I've never done it before but there are lots of instructions, AND I'm what we call "YouTube Certified". :bd:

I just figure that if I'm pulling it apart anyway it might be a good idea/make things a little happier to swap out the gears while I'm there.

I can't decide if the massive drop in vacuum over 60-65 is because of aero, which comes into play bigtime over 60, and I know that, or if it's just running out of grunt and would benefit from some steeper gears.

I mean, it's not like I'm dropping 2.73s in there, and some people wheel stock '80s Wagoneers, which have less torque and 2.73s...

[Stuka]

Having owned an FSJ with 2.73's, they are absolutely terrible. On and off road. My J10 had a crawl ratio of 18 when it was stock. I literally needed low range to drive up some peoples driveways (granted it was a manual).

And true, you would need to know the ramp speed of each gear to make a fully informed decision. And its a much bigger deal than RPM at speed. But, at 70mph, the RPM difference between 3.31 and 3.54 is 200rpm(ish) with 235/75R15 tires.

Also, those axles with 3.31's, those must be 80+? Which means the front axle is driver drop, and the rear is centered. So they won't even bolt up for you. The front is the wrong drop, and the rear will hit the fuel tank. No 70's FSJ came with 3.31s. Possible somebody installed them, but that would be abnormal as most want lower gears, not higher.

70's axles are 3.07 and 3.54 for Wagoneers.

[sierrablue]

They're brand new, uninstalled Yukon gears for a D44. I intended to use them with the J10 axles I got for the '88 but they've never been installed. One set is for a D44 and the other is for an AMC 20. Obv. the AMC isn't usable on my D44s. Never installed, still in the box.

According to what I've read, 3.07s were standard and 3.31s were optional, as were 3.73s and maybe 3.54s in my '71.

That 200 rpms is about 162,000 revolutions on tbe 13.5 hour drive one way I'd be taking it on, a few times a year. It doesn't seem like much but it adds up in a darned hurry.

I wasn't suggesting that the 2.73s were a good idea; I was saying that they're still usable with 2.73s and an emissions special 360. My B350 making more torque and having 3.31s should be fine based on that.

[Stuka]

Ahh, I had misread your post. I thought you had another set of axles with 3.31's. Not a stand alone set of gears. So disregard my comment on axles not working.

As far as I know, 3.31's became an option in '80. For '71 it would be 3.07 standard, 3.54 optional. 3.73, 4.10, and 4.56 were available for J-Trucks. Perhaps 3.73 for i6 equipped Waggys. I would have to dig out a book to check.

Either way, you have one set of gears, and if you are doing the labor yourself, it just comes to spending a day doing the rear axle. Personally, I would not find it worth while. But I am also not going on a 13 hour drive.

[Srdayflyer]

depends on tire size, my cherokee came stock with 2.73 gears ased on factory h78x15 tires, someone put 31x10.5x15 which dropped that ratio to effective rario 1.85 i installed a 4" spring lift to install 32x11.5x15 BFG's which would take it to 1.73 ratio, there are numerous on ratio charts that helped me determine my new ratio to give me the same performance as the original ratio that ratio was 3.73 there is a lot to consider when selecting gears besides just lifting the truck and buying tires,i also added eaton posi in the rear, also consider what lockers do and how they operate compaired to all other options, just my pointers to consider

[sierrablue]

Ahh, I had misread your post. I thought you had another set of axles with 3.31's. Not a stand alone set of gears. So disregard my comment on axles not working.

As far as I know, 3.31's became an option in '80. For '71 it would be 3.07 standard, 3.54 optional. 3.73, 4.10, and 4.56 were available for J-Trucks. Perhaps 3.73 for i6 equipped Waggys. I would have to dig out a book to check.

Either way, you have one set of gears, and if you are doing the labor yourself, it just comes to spending a day doing the rear axle. Personally, I would not find it worth while. But I am also not going on a 13 hour drive.

Thanks, Stuka. Sorry for the confusion.

And I'm not 100% on those gear ratios; that's just what I read in one questionably reliable Jeep book. Not a factory book or anything.

[sierrablue]

depends on tire size, my cherokee came stock with 2.73 gears ased on factory h78x15 tires, someone put 31x10.5x15 which dropped that ratio to effective rario 1.85 i installed a 4" spring lift to install 32x11.5x15 BFG's which would take it to 1.73 ratio, there are numerous on ratio charts that helped me determine my new ratio to give me the same performance as the original ratio that ratio was 3.73 there is a lot to consider when selecting gears besides just lifting the truck and buying tires,i also added eaton posi in the rear, also consider what lockers do and how they operate compaired to all other options, just my pointers to consider

As was said previously, in the title even, I'm asking about for my application; I know that it depends on tire size and such.

I'm running 3.54s right now on 235/75r15s. Considering going to 3.31s to help it out with long cruising, as the diffs need attention anyway, BUT I'm also going to be driving at high altitude and am concerned that I won't have the torque to deal with the 3.31s vs. 3.54s. I realize that an overdrive would be a better option, but I have no intention of investing that much in a gasoline drivetrain.

I have no desire to lift it or put tires on that are any bigger than 30x9.5s. My mountain bike is for my adventures.

@Herk, I know you've got some experience with your B350 at altitude based on your IFSJA posts from awhile ago--what do you think?

[Srdayflyer]

aspirated (carb), the factory used and altitude compensated carb on later models , timing also plays into , thats why i went with the holly sniper f.i system, and hyper spark ignition as they work together, as mixture is maintained at 14.1 optimum fuel ratio constantly maintained and ignition is constantly changing thru the f.i. computer ive had my rig upto 8000 feet and it runs the same as 890 feet but you will have to change carb jetting as you change altitude you could jet the carb for s.l. to 5-6000 feet and a jetting from 5-6000feet to 12K but the lower jetting will run rich at high altitudes no matter what. the altitude compensated carb have a bellows that adjusted mixture.

[sierrablue]

I have an Edelbrock tuning kit; jets, metering rods, etc. I'm not worried about that. And I can advance the timing to straighten things out if needed too.

However, it is physically impossible for it to be running the same at high altitude. There's less O2 in the air--it can't make the same amount of power with the same amount of throttle; there's just not enough oxygen in the air for that. It gets more extreme around 10k feet, but it is a noticeable difference before that.

Doesn't really matter about the altitude compensating carbs for my application, as I have a B350 and am not spending the money on an altitude compensating Qjet. And it's a Kaiser Wagoneer--I don't have an AMC V-8 and never have. What they have for carbs doesn't do me any good.

My question was whether or not I'd still have the torque to deal with 3.31s at altitude. I've already asked a billion questions about the carbs, and I'm sure I'll figure that out.

[akguy09]

I live at 7000 feet and I can tell you that if you have enough power at your current elevation, I wouldn't mess with the gears. Elevation kills more power in a Carbed engine then people realize and the gearing will help make up for it. If you plan on being above 10,000 i definitely wouldn't mess with the gears

[sierrablue]

I live at 7000 feet and I can tell you that if you have enough power at your current elevation, I wouldn't mess with the gears. Elevation kills more power in a Carbed engine then people realize and the gearing will help make up for it. If you plan on being above 10,000 i definitely wouldn't mess with the gears

OK. Thank you.

Definitely a noticeable difference in power yesterday when there was thick wildfire smoke in the air coming down from Canada. The particulate obv wasn't getting into the engine, but there was just less O2 available given the higher quantity of CO2, causing the same effect as some elevation.

I'm also thinking more and more that the major thing sucking vacuum (blowing vacuum???)(causing the vacuum to drop) maintaining speeds over 60 is the massive wall of air over 60 mph, which on this power scale is the point where aero starts making a noticeable difference in performance.

Thanks again for the insight.

[Herk]

Point of reference, my 71 J-4700 has 4.10 gears (33" tire). I've driven it from Boise to Texas, Little Rock (twice) and the Twin Cities. It was my daily driver for over 15 years and I've put well over 150K miles on it. Highway RPM is typically in the 3000-3300 RPM range (70-75 MPH). Fuel consumption is reliably 12 MPG city, 12 MPG highway. Never gave it another thought. My 83 Wagoneer had 3.31 gears and after I lifted it and put 31" tires under it I generally wished it had 3.73 gears, but sold it before I got that far. Fuel economy was 14-15 MPG. My current daily is an 05 LJ Rubicon with 4.56 gears and 33" tires. I commuted from Boise to Reno with it at least monthly for 12 years and never gave it much thought Fuel consumption is 16-17 MPG, and the posted speed limit is 80 MPH most places out here. Fuel economy was 14-15 MPG with the 33"s and the original 4.10 gears. So I'm definitely of the "don't fear the gear" school of thought. Manifold pressure can have at least as much effect on fuel economy as RPM, if not more under certain circumstances.

Here's a few thoughts I have in no particular order:

The stock gearing for a B350 powered Wagoneer was 3.31 with 3.73 optional per my 1970 FSM. 230 powered Wills Wagons were 3.54 or 4.09 Are you running earlier/later axles?

Never seen worn out spider gears unless everything else is worn out as well. Anything is possible I suppose.

If you are planning an EV powertrain swap in the future, you will almost certainly need to revisit the gearing then, so why mess with it now.

There is a "carrier break" on DANA axles, meaning different ratios might require a different differential carrier. IIRC it's 4.10 and lower vs. 3.73 and higher, at least on the 60, but I don't recall on the 27, 30 and 44.

If the ring/pinion gears are in reasonable condition, my experience has been that replacing all 4 bearings with name brand parts and reinstalling the shims in their original locations should get you real close pattern wise although lash might be a tad loose. Generally shimming the pinion "in" towards the differential slightly brings everything into spec, but the contact pattern dictates where to start.

Not knowing exactly what is "unhappy" about your Wag on the highway now, but if it's noise and vibration, start with at least 40 oz felt pad under the carpet. This will help with the noise and heat (what Jeep did). Leaking weatherstripping is also a huge noise maker. If vibration is an issue, start by making sure everything is balanced and running true. I start from the bottom up. That is, make sure the differential pinion bearings are in good shape and properly preloaded. Same for the output bearing on the transfer case. Then replace the u-joints, and have the shafts spin balanced. Pay attention to rear u-joint angles. This is a common source of vibration and bearing wear. We're assuming the wheel bearings are in good condition, adjusted correctly and the tires are round, balanced and all pointing in the same direction.

Engine noise/vibration and harshness (NVH) increases with RPM, but there's a lot of low hanging fruit here too. OEM style snorkel air cleaner, clutch fan, as well as a quiet muffler on a properly hung exhaust system are good places to start. Same with motor mounts, with the rear being the most likely to be junk (fix this before trying to adjust u-joint angles). If the transmission transfer or exhaust pipe touches the frame or body, it'll make a heck of a racket.

These vehicles were known for being pretty civilized on the highway when new, and this was before the 55 MPH speed limit (65-70 was typical). Fuel was cheaper then, but a lot of the things done in the 70's to regain fuel economy like super tall gearing mostly hurt performance and didn't help economy much.

As others have said, pulling 3.54 gears to swap in 3.31's wouldn't otherwise be worthwhile. The decrease in noise and performance would be slight but noticeable. If the word "trailer" is even a distinct possibility, you want to go the other way.

OK, that's one cup of coffee worth of "advice." I'll have to go back and re-read your thread to become more familiar with your Jeep. As I remember it's a pretty clean, complete, unmolested example. Never thought elevation was much of a factor in Minnesota (IIRC the field elevation at KMSP is something like 800 feet MSL), but elevation tuning is going to be another discussion.

[sierrablue]

I have a '74-76 front axle that was installed before I got it; rear is a flanged D44; I think it's the stock one with 6-lug shafts swapped into it, but I couldn't swear that it wasn't out of a '74ish too. My suspicion is that 3.54s were put in the back whenever the front axle swap was done, unless they were swapped together, in which case they're probably both stock mid-'70s axles.

Everything in the back looked fine when I changed the fluid and replaced the rear wheel bearings. I couldn't swear that the pinion/carrier bearings are still good, and the pinion seal on the rear axle does drip a little. Front wheel bearings were just repacked and look good, although I have new ones ready for it. I say the spider gears are worn out in the front because I can feel the gears in the front diff have the tiniest little bit of play, like you would expect, but the axle shafts don't start spinning until like 5° or so rotating where the u joint goes. The rear has some slop in it too but I can't tell if it's the tcase or axle side of things. It occasionally makes irritating drivetrain-type vibrations like it needs a u joint, but the u joints are fine. Front has locking hubs so right now they're not spinning anyway. I haven't been able to figure anything else out for it.

I think it's 3.73 down or 3.73 up numerically. Not 100% on it tho.

It is pretty civilized now. I just noticed that the vacuum takes a pretty good hit at about 65. If it was gonna maintain the torque it has now at 10k feet, I wouldn't hesitate to install the 3.31s, but as it stands, I couldn't decide.

In MN it's not a problem. Next year is college out west where there are real mountains lol

[Herk]

What you describe, being able to turn the driveshaft a few degrees is pretty much normal, and almost certainly not just the spider gears. It's the lash between the ring and pinion gears plus the spider gear lash. I wouldn't worry about it unless it's like a quarter turn. Spider gears really don't wear. Unless you are making a sharp turn they are along for the ride and aren't turning at all, or turn very little relative to each other, and even then they carry no load (unless one wheel is spinning on ice or something, and then they only carry as much load as it takes to spin the tire on the slippery surface which is obviously less than required to move the vehicle). On the highway they aren't turning at all when going straight ahead. Even if they were heavily worn, they wouldn't cause vibration. I would suspect the pinion bearings or the output bearing on the transfer case if it's not a u-joint. To diagnose you need to put a dial indicator on the yoke and try to move it up or down. Any movement at all indicates a worn or improperly preloaded bearing. Make sure the angles are correct, especially if the axles have been swapped. Mismatched u-joint angles will cause vibration and will wreck the pinion and output bearing surprisingly quickly. I'd replace the pinion seal, make sure the yoke nut is properly torqued and worry about other stuff if the bearings have no sideways play.

As for the manifold vacuum dropping at 65, that's to be expected. Think about it. Manifold vacuum is an indicator of engine load. It takes more power to stuff that brick through the air the faster you go. Parasitic drag increases exponentially with speed. As for torque loss with altitude, it's a thing, but I've never had a big issue. Will you regularly be driving 65 at 10,000 feet? They sold a lot more Wagoneers in Colorado than in Minnesota and they did just fine. Also, with a higher density altitude the drag on the vehicle is less, somewhat offsetting the loss of performance.

[Yeller]

Herk is right no matter what gearing you run vacuum is going to drop like a rock over 65. Wind resistance is not linear, it is expedential, it curves steeper and steeper the faster you go.

My bronco for instance…. At 55 gets 18mpg at 65 gets 12 at 75 gets 10 and is hard to keep in overdrive. Shift out of over drive and it gets 11. In OD it sits at 2150rpm at 75. In 3rd(direct) it at 2750rpm and using less fuel.

[sierrablue]

I understand about the aero; that's why I was asking if the steeper gears would actually help with that or not.

Also I wasn't blaming all my noises on the spider gears; the front end isn't even locked in rn, so there's no way those spider gears are causing me grief, other than having to haul them around with me all the time.

Out of curiosity, how much work is it to replace the pinion bearing? Obv the driveshaft needs pulled, and then can I just pull the yoke and leave the cover alone? Or do I need to pull the axle shafts a little ways, and the carrier and everything?

Been thinking I should rebuild the D20, replacing all the seals and bearings, and filling it with full synthetic oil...more slippy slidy=better mileage