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Made A Discovery, Looking For Some Advice[Gearing, Tire Size Related]


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Hello All,

 

A little info up front, I have a 2.5 TBI, AX5 Transmission, Command Trac 207 transfercase LWB Comanche.

 

Since I'm an idiot, I just assumed my diffs were both factory spec still. I ignored the sand I found in the weirdest places, the offroad tires, all of it. I assumed both diffs were 4.10/11 as they should be from the factory. Well, I found out they're not. Someone actually spent some money on my Comanche and had both axles regeared to 4.56. Not only that, I have some sort of weird Locker/LSD diff in there. It's not a Detroit locker, it's not an Eaton. I can't find a brand stamped on it anywhere. It's just a weird contraption.

 

I can only assume some people would be happy with this discovery, means that I can fit bigger tires. In fact, some quick googling turned up that to make the most of 4.56 I need 33 or 35'' tires. I'm not happy with it at all. I never wanted 33's, let alone 35's. This car is meant to be 80-90% highway useage for me. 

 

This is why I'm writing this, I'm looking for some advice.

I already scored Jeep Icon rims off of a 2000 limited, I've already selected new tires I want. I was going to get 225/70/R16 (about 28.5'' tires). Obviously my current gearing is wrong for that tire size, it explains why my gears are so damn short. It's been driving like an old diesel. The revs on the highway have also been pretty crazy. Not only that, the ''lockers'' are also failing. They're just kaput, so not only do I need regearing, I need entire new diffs.

 

Problem number one, what gearing do I go to now? Do I want to go back to stock 4.10?

 

Running the numbers through Grimmjeep's calculator, with my current tire size all in fifth gear:

 

(235/75/15) and 4.56 I get 3157 RPM at 70 Mph.

(235/75/15) and 4.10 I get 2838 RPM at 70 Mph.

(235/75/15) and 3.55 I get 2457 RPM at 70 Mph.

 

With my newly desired tires:

 

(225/70/16) and 4.56 I get 3210 RPM at 70 Mph

(225/70/16) and 4.10 I get 2886 RPM at 70 Mph

(225/70/16) and 3.55 I get 2499 RPM at 70 Mph

 

I would think that 2499 is the sweet spot, 3.55 is also a common gearing. But what does that do for my driving experience? I have a heavy foot, I know the 2.5 isn't much but I like to feel of a little pushback into my sweet when I accelerate. Obviously with 4.56 that's not present right now. How do I regain that? The fact that it came with 4.10 from the factory has to have a reason, but I just don't know what would be best for me.

 

I welcome any insight.

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The nice thing about picking 3.54/55 is, I can just go out and find a 8.25 Chryco and put that in. No need for finding a full diff, it's stronger and I can mount disc brakes in the future. I already have the correct backing plates. I just don't understand why Chrysler would put 4.10 in from the start. If only I could drive a jeep with 3.55 and feel the difference, to just be sure of what I'm about to do.

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If 235's with 4.56's feels slow... either pick up a 2wd 4.0 5 speed and sell your 4 banger, or start thinking about an engine swap. 3.55's WILL NOT make it accelerate faster.

:agree:  a gutless motor is still a gutless motor...225/235 with 3.55 is extremely common. I still don't think it will give you what you want. IMHO sounds like an LS swap is in your future. haha :yes: ;) just kidding....no but seriously.

 

Seems like you have plan to keep it as a street truck- you can still run somewhat oversized tires on the highway and still get good MPG. The best highway MPG I ever got in my cherokee was when I put on some 31s with the factory 3.55 gears. Now on my MJ with 3.07 and 31s....yeah it sucks but its doable. 

 

 

Check out this chart maybe it will help some: http://www.offroaders.com/tech/gear-ratio-chart.htm

And check this out if you want to get fancy: http://www.grimmjeeper.com/gears.html

 

That's just my two cents and I hope it helps out some.

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The nice thing about picking 3.54/55 is, I can just go out and find a 8.25 Chryco and put that in. No need for finding a full diff, it's stronger and I can mount disc brakes in the future. I already have the correct backing plates. I just don't understand why Chrysler would put 4.10 in from the start. If only I could drive a jeep with 3.55 and feel the difference, to just be sure of what I'm about to do.

After having many 4.0 MJs, I was concerned about 3.55s in my newly acquired 2.5 truck.

 

It was fine with the 235s. 

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I haven't quite figured out what i want to do engine wise. Swapping in a 4.0 might be in the future, provided I can find a donor car for the right money. It's just so hard to find an AX15/4.0 combo where I live. I may just keep the 2.5, too. it's a good engine, no issues and starts every time. *knock on wood.

 

I'm going to see what I can get. I'll talk to my local Jeep shop monday, they have a 2001 XJ up for parts. If that has 3.55 and the price is right, I may just go for that. Then I can start worrying about the front axle.

 

Thanks for replying so far, it's definitely a luxury to have a forum full of experienced Comanche owners.

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Power and of the 4cyl (optimum fuel mileage range) is 3200-3400 rpm or so. You are fine running 4.56, and should leave it alone. You honestly should go with 235, though. It will give you 80mph with no issues.

 

Don't be afraid to run it in the higher rpm range. It is what it was built for.

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Power and of the 4cyl (optimum fuel mileage range) is 3200-3400 rpm or so. You are fine running 4.56, and should leave it alone. You honestly should go with 235, though. It will give you 80mph with no issues.

 

Don't be afraid to run it in the higher rpm range. It is what it was built for.

^^^ This. The tires you want to run (225/70-16) are exactly the same size as 225/75-15. 3.55 gears were what the factory used with that tire size behind a 4.0L with automatic. You need more RPMs to be able to run a 2.5L at 70 MPH.

 

According to my calculations (which are more accurate than most on-line speed-to-RPM calculators I've seen, with 225/70-16 tires you would get the following at 70 MPH:

 

3.55 ... 3040 RPM in 4th gear, 2280 RPM in 5th gear

 

3.73 ... 3203 RPM in 4th gear, 2402 RPM in 5th gear

 

4.10 ... 3521 RPM in 4th gear, 2640 RPM in 5th gear

 

4.56 ... 3916 RPM in 4th gear, 2937 RPM in 5th gear

 

In reality, you're fine with the 4.56 gears you have. What too many people forget is that the basic design of the AMC engines (both the 4.0L and the 2.5L, since both are derived from the older 232 cubic inch I-6) dates to long before overdrive transmissions were commonly used. In the 1960s and early 70s, AMC cars came from the factory geared to run approximately 24 MPH per 1000 RPM. That worked out to 2500 RPM at 60 MPH, and 3000 RPM at 72 MPH. And that wasn't "burning the engine up." My brother had a 1970 Gremlin with a 3-speed manual tranny and the 232 c.i.d. engine that went over 300,000 miles. And he used it to win a couple of state autocross championships as well as being a daily driver.

 

So the 4.56 gearing puts you right where you should be for the 2.5L. You could also get by with 4.10s, but I think you would be horribly unhappy with any less gear than 4.10s.

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^^^ I agree with that as well. I started out with a 4 speed and 3 :55's, switched to 4 :10's. That was with 235's. It ran great with the 4 :10's, though I wish I had a little more, and your 4 :56's would be about perfect, especially now that I added JK take-off wheels and tires at 32". But still, even with 235's I wish I had more. I think youll be dissappointed in your power if you go to higher gears. And if you go to 3 :55, your fifth gear will be all but useless as you won't have enough power from the 2.5 to make use of it at those speeds.

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Hope he never pulls a trailer, never drives up hill, never carries any weight.

 

But then........you guys don't drive them as trucks........apparently.

 

The lower gears are numerically higher. That negates your statement, as the lower the gear, the more torque transferred to the tires, the better the pulling power

 

 

You make it sound like we are telling him to run 2.72 gears on a stock set of tires. That would put him at 4th gear being his last useful gear somewhere cerca 90mph, and his starting gear seeming like starting in 3rd gear with stock tires and gears.

 

 

Rethink your understanding of gear ratios

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Hope he never pulls a trailer, never drives up hill, never carries any weight.

 

But then........you guys don't drive them as trucks........apparently.

 

The lower gears are numerically higher. That negates your statement, as the lower the gear, the more torque transferred to the tires, the better the pulling power

 

 

You make it sound like we are telling him to run 2.72 gears on a stock set of tires. That would put him at 4th gear being his last useful gear somewhere cerca 90mph, and his starting gear seeming like starting in 3rd gear with stock tires and gears.

 

 

Rethink your understanding of gear ratios

 

 

 

 

Suppose you were alone and you had to move a 1000lbs safe. You had a 2' bar and a 6' bar to use as levers. The 2' bar would require more strength, more energy, more EFFORT, the 6' bar would require less effort than the 2' bar to do the same amount of work.

Same with gearing.

The 2.72 gear would require more work from the engine to move the truck the same distance as a 4.11 gear would, the consequence is that you run out of RPMs earlier, as is his complaint.

He has a 2.5 which has less energy or available effort to begin with, the 4.56 gear allows his engine to work less, if he were pulling a 3000lbs trailer up a hill he'd have more torque available to do the work.......ie.........longer lever.

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If the OP were local to me, I'd have him come over and swap out the 31s off mine to his rear, let him drive around for a hour and see how it does.

 

If he is otherwise happy with the truck, handles well, brakes well....etc... Why go through the expense of swapping axles when all he may have to do is choose a different tire size?

 

Consider all........bearing, seals, brakes, axle u-joints, steering links, control arms, axle u-bolts, e-brake cables....etc........all the parts you will be dealing with or effecting.......drive shaft length and so on.......(he has yet to state what rear axle he has and what he may swap to).

 

 

 

Pick up a set of used wheels with 31s for $300 and see how you like it first.

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Hope he never pulls a trailer, never drives up hill, never carries any weight.

 

But then........you guys don't drive them as trucks........apparently.

 

The lower gears are numerically higher. That negates your statement, as the lower the gear, the more torque transferred to the tires, the better the pulling power

You make it sound like we are telling him to run 2.72 gears on a stock set of tires. That would put him at 4th gear being his last useful gear somewhere cerca 90mph, and his starting gear seeming like starting in 3rd gear with stock tires and gears.

Rethink your understanding of gear ratios

 

 

Suppose you were alone and you had to move a 1000lbs safe. You had a 2' bar and a 6' bar to use as levers. The 2' bar would require more strength, more energy, more EFFORT, the 6' bar would require less effort than the 2' bar to do the same amount of work.

Same with gearing.

The 2.72 gear would require more work from the engine to move the truck the same distance as a 4.11 gear would, the consequence is that you run out of RPMs earlier, as is his complaint.

He has a 2.5 which has less energy or available effort to begin with, the 4.56 gear allows his engine to work less, if he were pulling a 3000lbs trailer up a hill he'd have more torque available to do the work.......ie.........longer lever.

I have been standing here re reading this. Go back and read all my posts. We. Are. Saying. The. Same. Thing.

 

The only thing we are not agreeing on is rpm, and tire size. The 2.5 is made to run over 3000. 70mph is about the best you could expect from one.

 

The gears are perfect. 225 or 235 are perfect.

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I would think that 2499 is the sweet spot, ...

You can think that but, as most of us have commented, you would be wrong.

 

I have a heavy foot, I know the 2.5 isn't much but I like to feel of a little pushback into my sweet when I accelerate. Obviously with 4.56 that's not present right now. How do I regain that?

You can't regain what you never had, and the 2.5L Jeep 4-banger is simply never going to push you back in your seat like a 5-liter Mustang. You can get a little bit of acceleration feel in first and second gear, beyond that acceleration is "gradual," at best. That's just the nature of the beast. The only way to get more feel of acceleration is with MORE gear or smaller tires. You certainly won't get it by changing from 4.56 gears to 3.55s.

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Yup 2499 would be the good if it were a 4.0L but not the 2.5L...

 

The 2.5 is a Rev-er and in stock trim is 2800RPMish as you already noted earlier so the ideal RPM is to get your new numbers closer to the RPMs the engine is best running at plus or minus a few depending on your purpose for the vehicle...

 

I am no fan of the 2.5L but if its gutless with the 4:56's then I think you may have bigger issues then gearing...

 

Oh and a Comanche should never be referred to as a "CAR"

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First off, thank you all for replying. I didn't think a gearing thread would get this much attention, since there's one every other day or so. I appreciate every reply.

 

Power and of the 4cyl (optimum fuel mileage range) is 3200-3400 rpm or so. You are fine running 4.56, and should leave it alone. You honestly should go with 235, though. It will give you 80mph with no issues.

Don't be afraid to run it in the higher rpm range. It is what it was built for.

 

 

 

Power and of the 4cyl (optimum fuel mileage range) is 3200-3400 rpm or so. You are fine running 4.56, and should leave it alone. You honestly should go with 235, though. It will give you 80mph with no issues.

Don't be afraid to run it in the higher rpm range. It is what it was built for.


^^^ This. The tires you want to run (225/70-16) are exactly the same size as 225/75-15. 3.55 gears were what the factory used with that tire size behind a 4.0L with automatic. You need more RPMs to be able to run a 2.5L at 70 MPH.

According to my calculations (which are more accurate than most on-line speed-to-RPM calculators I've seen, with 225/70-16 tires you would get the following at 70 MPH:

3.55 ... 3040 RPM in 4th gear, 2280 RPM in 5th gear

3.73 ... 3203 RPM in 4th gear, 2402 RPM in 5th gear

4.10 ... 3521 RPM in 4th gear, 2640 RPM in 5th gear

4.56 ... 3916 RPM in 4th gear, 2937 RPM in 5th gear

In reality, you're fine with the 4.56 gears you have. What too many people forget is that the basic design of the AMC engines (both the 4.0L and the 2.5L, since both are derived from the older 232 cubic inch I-6) dates to long before overdrive transmissions were commonly used. In the 1960s and early 70s, AMC cars came from the factory geared to run approximately 24 MPH per 1000 RPM. That worked out to 2500 RPM at 60 MPH, and 3000 RPM at 72 MPH. And that wasn't "burning the engine up." My brother had a 1970 Gremlin with a 3-speed manual tranny and the 232 c.i.d. engine that went over 300,000 miles. And he used it to win a couple of state autocross championships as well as being a daily driver.

So the 4.56 gearing puts you right where you should be for the 2.5L. You could also get by with 4.10s, but I think you would be horribly unhappy with any less gear than 4.10s.

 

 

I read this right before leaving work, and decided to just go for it after work. I've been chasing a death wobble as well as a leaking exhaust, so I've been a little timid to really get up to 80 Mph but I decided that I needed to try this out. I took my Comanche out on the highway and just kinda went balls to the wall with it. My rev counter isn't calibrated yet, but I went past my comfort zone and I discoverd a power band of sorts. Where third/fourth was lackluster before, if I revved it enough I could definitely feel it pull. Even going from 65 to 75 it would pull, instead of crawling up to speed. I have just been looking at all of this wrong, because I keep thinking my 2.5 is a shorter 4.0...expecting it to have the same characteristics and power band. 

 

If the OP were local to me, I'd have him come over and swap out the 31s off mine to his rear, let him drive around for a hour and see how it does.

 

If he is otherwise happy with the truck, handles well, brakes well....etc... Why go through the expense of swapping axles when all he may have to do is choose a different tire size?

 

Consider all........bearing, seals, brakes, axle u-joints, steering links, control arms, axle u-bolts, e-brake cables....etc........all the parts you will be dealing with or effecting.......drive shaft length and so on.......(he has yet to state what rear axle he has and what he may swap to).

 

 

 

Pick up a set of used wheels with 31s for $300 and see how you like it first.

 

Thank you for the offer! I would take you up on it if there weren't an Ocean between us. I live in the Netherlands. Also, it's a Dana 35 in the back(which is why I wanted to swap to a chryco 8.25)

 

 

 

 

I would think that 2499 is the sweet spot, ...


You can think that but, as most of us have commented, you would be wrong.

I have a heavy foot, I know the 2.5 isn't much but I like to feel of a little pushback into my sweet when I accelerate. Obviously with 4.56 that's not present right now. How do I regain that?


You can't regain what you never had, and the 2.5L Jeep 4-banger is simply never going to push you back in your seat like a 5-liter Mustang. You can get a little bit of acceleration feel in first and second gear, beyond that acceleration is "gradual," at best. That's just the nature of the beast. The only way to get more feel of acceleration is with MORE gear or smaller tires. You certainly won't get it by changing from 4.56 gears to 3.55s.

 

 

Exactly, i am wrong. it's not a 4.0. Though I must say, now that I've tried revving it higher it does give me a nice push back, even in fourth.

 

Oh and a Comanche should never be referred to as a "CAR"

I was wondering if someone was going to catch that, I'll refrain from using ''Car''.

 

To summarise, I've taken it out on the highway, revved it higher than I was doing before, and found satisfying results. My deathwobble/exhaust is still keeping me from really trying to gun it, but I've decided(all thanks to all of you) to keep my 4.56. I still need to return to the open diff style, and both yokes are worn. But this should save me greenbacks.

 

Thank you all.

 

 

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I read this right before leaving work, and decided to just go for it after work. I've been chasing a death wobble as well as a leaking exhaust, so I've been a little timid to really get up to 80 Mph but I decided that I needed to try this out. I took my Comanche out on the highway and just kinda went balls to the wall with it. My rev counter isn't calibrated yet, but I went past my comfort zone and I discoverd a power band of sorts. Where third/fourth was lackluster before, if I revved it enough I could definitely feel it pull. Even going from 65 to 75 it would pull, instead of crawling up to speed. I have just been looking at all of this wrong, because I keep thinking my 2.5 is a shorter 4.0...expecting it to have the same characteristics and power band.

Mechanically, both engines were developed from the same in-line 6-cylinder predecessor, but they are very different. The original 4.0L had a torque peak of 220 foot-pounds at 2000 RPM in 1987, and then 224 foot-pounds at 2400 RPM for 1988 through 1990. But for the 4.0L the term torque "peak" is misleading, because the torque curve beyond the peak remains almost flat all the way up to about 4000 RPM. On the other hand, for the 2.5L the curve has a very clearly defined peak. For 1986 the 2.5L produced 135 foot-pounds at 3500 RPM, and for 1987 through 1990 it produced 141 foot-pounds at 3250 RPM. "Acceleration" is produced more by torque than by horsepower, so to feel acceleration in the seat of your pants you need to keep the engine running near the torque peak.

 

Here's one torque curve for the 4.0L (This one is for an HO version but it's all I could find). Notice how basically flat it is all the way from 1000 RPM to 5000 RPM.

tcurve.gif

 

Here's a torque/horsepower chart for the 2.5L engine. Compared to the 4.0L torque curve, note how the torque rises toward the peak and then falls off after the peak, with a very obvious spike right at the peak of 3200 RPM:

power-25.gif

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