Lower 5th gear ratio for AX-15?

Yasha

Yasha

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AX-15 gear ratios are 3.83, 2.33, 1.44, 1.00, 0.79. With 4.10 axles and 31" tires (currently building axles so this is a far off plan), I could do with a taller 5th gear ratio. There isn't any obvious off-the-shelf solution, but what if I got two donor transmissions and used a 3rd gear set up with the gears flipped for 5th? That would give a 0.694 ratio in 5th gear. Having built other transmissions where the synchronizers are pressed onto the axles in front of the gears, I see no reason this wouldn't be possible if the AX-15 is built the same.
 
Yasha said:
AX-15 gear ratios are 3.83, 2.33, 1.44, 1.00, 0.79. With 4.10 axles and 31" tires (currently building axles so this is a far off plan), I could do with a taller 5th gear ratio. There isn't any obvious off-the-shelf solution, but what if I got two donor transmissions and used a 3rd gear set up with the gears flipped for 5th? That would give a 0.694 ratio in 5th gear. Having built other transmissions where the synchronizers are pressed onto the axles in front of the gears, I see no reason this wouldn't be possible if the AX-15 is built the same.
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I'd keep it as-is. Rubicons come stock from the factory with 4.10 and 31" tires.
 
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Jerry Bransford said:
I'd keep it as-is. Rubicons come stock from the factory with 4.10 and 31" tires.
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That assumes the stock Rubicon came from the factory with appropriate gearing. Remember the Rubicon came with three different transmissions, all with the same 4.10 gears. Which one was correct? Were any of them correct? And why was the 32rh geared for higher rpms than any other TJ?
 
Pretty sure the counter third gear is cast and machined into the countershaft/cluster gear. You'd have to find something with the counter 5th gear spline arrangement but the third gear tooth count.

Screenshot_20220625-141355.png



Then for the main gear, the output shaft is stepped so the 3rd gear probably wouldn't ride on the 5th gear journal without a sleeve to take up the space. And I have no idea whether the other interfacing parts would mate up.

Screenshot_20220625-142210.png
 
I didn't consider that 3rd gear would be machined onto the countershaft in this box. In this case it would not work as a donor without considerable machining effort to extract that 3rd gear and that is probably extensive. I'm having a little trouble following how the 5th gears mate. On the mainshaft, it uses a needle roller bearing like the other gears, but on the counter shaft, it is a removable gear with splines that mate to the assembly of 1-4?

Here's a new idea. I did a little reading on toyota R series gearboxes (of which the AX-15 is a member, a rebrand for Jeep from what I read).

https://board.marlincrawler.com/index.php?topic=92791.0
https://www.pirate4x4.com/threads/gm-metric-v6s-on-your-ax15-or-r150f-in-jeep.667318/
https://www.supraforums.com/threads/63-overdrive-5th-gear-for-r154-for-those-that-dont-know.153443/
https://board.marlincrawler.com/index.php?topic=104378.0

Specifically they mention the R150 and R154 boxes having different 5th gear ratios (with R154 having 0.674 5th gear, almost exactly what I'm looking for). I'd bet maybe the 5th gear set would be a fit for an AX-15 since they were designed by Toyota in the same series more or less. But these R150 and R154 discussions mention a countergear ratio in addition to the input/layshaft ratio - I can't see enough of these boxes open to understand what this means. Does AX-15 have a set additional input ratio or is the ratio set by only the input/layshaft ratio? If it's the latter, maybe such a swap is possible if I can find donor parts from an R154.

@Jerry Bransford @jjvw The 42RLE automatic has overdrive ratio 0.69. Thus a Rubicon with an auto trans has 0.69 highest gear ratio and also has 4.10 axles and 31" tires. For the sake of discussion this implies the factory would have endorsed a taller 5th gear on AX-15. Why they didn't go this route, who knows. Who is to say what Jeep/Chrysler got right vs. left in dire need of adjustment?
 
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Yasha said:
I didn't consider that 3rd gear would be machined onto the countershaft in this box. In this case it would not work as a donor without considerable machining effort to extract that 3rd gear and that is probably extensive. I'm having a little trouble following how the 5th gears mate. On the mainshaft, it uses a needle roller bearing like the other gears, but on the counter shaft, it is a removable gear with splines that mate to the assembly of 1-4?

Here's a new idea. I did a little reading on toyota R series gearboxes (of which the AX-15 is a member, a rebrand for Jeep from what I read).

https://board.marlincrawler.com/index.php?topic=92791.0
https://www.pirate4x4.com/threads/gm-metric-v6s-on-your-ax15-or-r150f-in-jeep.667318/
https://www.supraforums.com/threads/63-overdrive-5th-gear-for-r154-for-those-that-dont-know.153443/

Specifically they mention the R150 and R154 boxes having different 5th gear ratios (with R154 having 0.674 5th gear, almost exactly what I'm looking for). I'd bet maybe the 5th gear set would be a fit for an AX-15 since they were designed by Toyota in the same series more or less. But these R150 and R154 discussions mention a countergear ratio in addition to the input/layshaft ratio - I can't see enough of these boxes open to understand what this means. Does AX-15 have a set additional input ratio or is the ratio set by only the input/layshaft ratio? If it's the latter, maybe such a swap is possible if I can find donor parts from an R154.

@Jerry Bransford @jjvw The 42RLE automatic has overdrive ratio 0.69. Thus a Rubicon with an auto trans has 0.69 highest gear ratio and also has 4.10 axles and 31" tires. For the sake of discussion this implies the factory would have endorsed a taller 5th gear on AX-15. Why they didn't go this route, who knows. Who is to say what Jeep/Chrysler got right vs. left in dire need of adjustment?
Click to expand...

The Toyota thing may be a possibility. I've come across references that indicate it may be possible to swap the countershaft and first from an r151f to get a 4.3:1 first gear, which is more attractive to me than a higher 5th since I just swapped an ax in place of an nsg370 and lost my very nice 4.46 first gear.

All that said, if you're after lower highway rpm I have to ask why? The 4.0 seems to like, and actually prefer being around 3000rpm at 75 and unless you have a torque v8 swap or some crazy deep axle gears already I'm not sure what you expect to gain from this.
 
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I'll gladly take a lower 1st gear as well, granted only 4.10 axles may give my 3.83 stock 1st plenty of torque for startup and pleasant low speed creeping - we'll see. As for taller 5th I'm mainly after a tiny drop in cruise RPM to make my girlfriend more comfortable on long road trips. I'd take the extra fuel economy too if possible. I didn't even realize the TJ was rated for up to 18 mpg highway when it was made. I see 12.8 city and 16 highway. Again not something I care about but it makes the better half happier if it's a little bumped.
 
The only reason I could possibly fathom wanting a taller OD would be so that you could gear the axles even deeper to keep the same cruise but have lower 1-4 for acceleration power and slower offroading. I can’t fathom changing the OD to be taller to lower the rpm/noise, it’s counter intuitive and in the case of a TJ will pretty much make driving it worse since they don’t have a bunch of power and need to rev.

I’m also of the opinion that the girlfriend will not know the difference. Jeeps are loud for pretty much 100 reasons but i can’t say that any specific rpm I rev down the highway whether it’s 2800 or 2400 really sounded much different either way. Most of the time the wind and tires were too loud and drowned out any inkling of engine noise. In other words, if you blindfolded a Jeep owner and made them ride passenger, with 3 different ratios, I doubt based on noise they’d be able to tell you which is which.
 
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Yasha said:
I'll gladly take a lower 1st gear as well, granted only 4.10 axles may give my 3.83 stock 1st plenty of torque for startup and pleasant low speed creeping - we'll see. As for taller 5th I'm mainly after a tiny drop in cruise RPM to make my girlfriend more comfortable on long road trips. I'd take the extra fuel economy too if possible. I didn't even realize the TJ was rated for up to 18 mpg highway when it was made. I see 12.8 city and 16 highway. Again not something I care about but it makes the better half happier if it's a little bumped.
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I would encourage you to try a similarly setup TJ with a 42RLE before you go to the trouble. Many consider it's OD too aggressive even with stock tires and gearing.

If you're running stock 30" tires with 4.1 gears and an ax15, and then do what you're saying, that would drop your rpm @ 75mph to under 2500, where the 4.0 is gutless. I had a 99 with AX15, 3.73 and 32's which was effectively almost identical to what you're proposing and I hated the highway until I regeared it. 5th gear was basically useless because I didn't have enough torque to accelerate, and by the time 4th got me to cruising speed, there was so much wind resistance that I had to downshift on even the slightest inclines.
 
jjvw said:
That assumes the stock Rubicon came from the factory with appropriate gearing. Remember the Rubicon came with three different transmissions, all with the same 4.10 gears. Which one was correct? Were any of them correct? And why was the 32rh geared for higher rpms than any other TJ?
Click to expand...

Because if they were to gear the 32rh to have the same highway rpm as the others then it would have no power down low, kinda like what happened on the 3.07 3 speeds. I’m of the opinion that 3000 at 75 is proper, but even the 3 speeds were significantly higher than that on 3.73 and 28’s, and also when folks regear them to 4.56 for 35’s. It’s more like 3350-3400 at 75 by then, obviously fluctuating some due to actual tire size. But I don’t think the general consensus is that the 5 speeds should be geared to do 3400 at 75?

I do agree choosing a ratio based on what stock did is misleading since there were a million “stocks”, and no way to choose which one specific stock is actually correct, however a manual on 4.10/31 is far from a bad setup to be stuck with at least. I’d definitely rather 4.56 for 31’s though.
 
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jjvw said:
It was difficult enough to get people to believe that 3k at 75mph wasn't going too low in gearing.
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I know, I’m just trying to make the point though that I don’t think 3400 at 75 would be better than 3000. I enjoy low gearing but I don’t see what benefit going that far provides. I’ve even seen a few other low-gear-lovers running 5.13/35/6speed who after a while have decided it’s too low (and wish for 4.88 to be back at 3k/75). Not that it hurts anything the way they have it but it’s just a little lower than is really necessary.

That was my only point, that in my opinion the 3 speed was geared the way it was out of necessity, not that it was necessarily the best simply because it revved the highest.
 
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I've been thinking about this a lot the past few weeks, notably after putting about 1000 highway miles on my Jeep. I think the engine performance is satisfactory for me above 2200, on the highway with a passenger and packed travel bags. Granted to get "that high" I need to go over 75. Also had some time in the hotel to do a lot of reading, looks like the Toyota R series gears give the ability to re-gear 5th in the AX-15 to 0.63 or 0.69. Compared to my current setup (3.07, 29" tires), gearing to 0.63 is utterly useless as it gets me back to the same equivalent final drive in 5th (with 4.10, 31" tires). But 0.69 still gives a slight boost to 5th gear highway rpm and should retain a nice little boost to highway performance.

Now while I could always go for 33s at a a modest (6%) drop in torque over 31s, that also drops the low end and I don't think I will like that. Obviously all this will have to wait until after I'm done with my axles. Kinda getting pooped out laboring in the garage for the time being and I'm waiting until my 29s are on their last legs of life so I can swap axles and justify needing new tires anyway.
 
Yasha said:
I've been thinking about this a lot the past few weeks, notably after putting about 1000 highway miles on my Jeep. I think the engine performance is satisfactory for me above 2200, on the highway with a passenger and packed travel bags. Granted to get "that high" I need to go over 75. Also had some time in the hotel to do a lot of reading, looks like the Toyota R series gears give the ability to re-gear 5th in the AX-15 to 0.63 or 0.69. Compared to my current setup (3.07, 29" tires), gearing to 0.63 is utterly useless as it gets me back to the same equivalent final drive in 5th (with 4.10, 31" tires). But 0.69 still gives a slight boost to 5th gear highway rpm and should retain a nice little boost to highway performance.

Now while I could always go for 33s at a a modest (6%) drop in torque over 31s, that also drops the low end and I don't think I will like that. Obviously all this will have to wait until after I'm done with my axles. Kinda getting pooped out laboring in the garage for the time being and I'm waiting until my 29s are on their last legs of life so I can swap axles and justify needing new tires anyway.
Click to expand...

Why not wait and see how the 4.10 and 31’s are first before doing all this brainwork? 4.10 and 31’s was a factory setup, so it’s not at all crazy…..do that first and then if you decide 5th is too low, go crazy with a 5th gear swap. I really don’t think you’ll want a 5th gear swap on 4.10/31’s.

Just wanting to make the point that you have it in your head that you won’t like your future 5th gear with 4.10’s, but you haven’t experienced it. It’s a lot better than I think you’re expecting it to be.

Now, I will say it would be cool to do a 0.69 swap and then gear extra deep like 4.88 and 31’s. Then you’d have a more powerful 1-4 and would retain the same 5th as 4.10 and 31’s with the 0.79 OD.
 
And to add to this discussion from a different angle, the 03-06 4-speed auto comes with a 0.69 overdrive. It also came stock with 4.10 and 31’s. That combo has tons of complaints about being too sluggish on the highway. So you might consider that before thinking you want to go that route….definitely do the axle gears and tires first and go from there. I really think after the regear you’ll scrap the whole transmission modding idea.
 
machoheadgames said:
Why not wait and see how the 4.10 and 31’s are first before doing all this brainwork? 4.10 and 31’s was a factory setup, so it’s not at all crazy…..do that first and then if you decide 5th is too low, go crazy with a 5th gear swap. I really don’t think you’ll want a 5th gear swap on 4.10/31’s.

Just wanting to make the point that you have it in your head that you won’t like your future 5th gear with 4.10’s, but you haven’t experienced it. It’s a lot better than I think you’re expecting it to be.

Now, I will say it would be cool to do a 0.69 swap and then gear extra deep like 4.88 and 31’s. Then you’d have a more powerful 1-4 and would retain the same 5th as 4.10 and 31’s with the 0.79 OD.
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Agree. I've never heard of anyone complaining that ANY TJ transmission had too little overdrive.
 
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machoheadgames said:
Why not wait and see how the 4.10 and 31’s are first before doing all this brainwork? 4.10 and 31’s was a factory setup, so it’s not at all crazy…..do that first and then if you decide 5th is too low, go crazy with a 5th gear swap. I really don’t think you’ll want a 5th gear swap on 4.10/31’s.

Just wanting to make the point that you have it in your head that you won’t like your future 5th gear with 4.10’s, but you haven’t experienced it. It’s a lot better than I think you’re expecting it to be.

Now, I will say it would be cool to do a 0.69 swap and then gear extra deep like 4.88 and 31’s. Then you’d have a more powerful 1-4 and would retain the same 5th as 4.10 and 31’s with the 0.79 OD.
Click to expand...

You're right, I'm doing a lot of brainwork before it's even necessary. I guess that's how I pass the time when I'm too tired to get out and wrench. 4.56 or 4.88 would be nice. I think I'll save pondering that for after my 4.10 swap too :)
 
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Yasha said:
I've been thinking about this a lot the past few weeks, notably after putting about 1000 highway miles on my Jeep. I think the engine performance is satisfactory for me above 2200, on the highway with a passenger and packed travel bags. Granted to get "that high" I need to go over 75. Also had some time in the hotel to do a lot of reading, looks like the Toyota R series gears give the ability to re-gear 5th in the AX-15 to 0.63 or 0.69. Compared to my current setup (3.07, 29" tires), gearing to 0.63 is utterly useless as it gets me back to the same equivalent final drive in 5th (with 4.10, 31" tires). But 0.69 still gives a slight boost to 5th gear highway rpm and should retain a nice little boost to highway performance.

Now while I could always go for 33s at a a modest (6%) drop in torque over 31s, that also drops the low end and I don't think I will like that. Obviously all this will have to wait until after I'm done with my axles. Kinda getting pooped out laboring in the garage for the time being and I'm waiting until my 29s are on their last legs of life so I can swap axles and justify needing new tires anyway.
Click to expand...

Do you have some sources on swapping the 5th gear into AX15? I am very interested in the .63 idea that would work well with my LS at highway speeds.
 
So sorry I just saw your message, I did some extensive research and forgot all about this last summer. Let me see what I can dig out... I for one have abandoned this idea simply because I got bored of it and would rather focus on lifting my Jeep and regearing axles.
 
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