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Yet another gearing question

R

Rob Mayercik

NAXJA Member #920
NAXJA Member
Location
NJ, U.S.A.
Ok, I know this has been asked before, and I have read several recent threads on the matter as well as CRASH's missive in the stickies collection above. That being said:

I have been running 30x9.50x15s for a couple of years now on my '92, using the stock axle gearing (3.55). It's not been all that bad, but at times it has felt like the engine was laboring a bit on hills (I have on occasion run around town in "3" instead of "D" to keep the rpms up, but stick with "D" on the highways). The tach reads about 2K rpms at 60mph. Never really thought much about regearing (expensive).

Now that I'm starting the process of going over the vehicle to prep it for the run to Moab in October, I've been once again thinking about the gears. While I don't expect any issues out in the plains, it's the mountain climbs nearer the west end of the trip that concern me - whether I am going to run into trouble being slightly undergeared relative to stock. As I am already at 30s, I figured 4.10s would be the most logical choice, since the notion of going up an inch in suspension and maybe going to 31s when the 30s wear out has been on my mind since NACFest.

Obviously, I have done a little research on having my existing gears replaced (instant sticker shock, just add water). The guy I spoke to about it (local mechanic/speed shop) suggested I call a junkyard that specializes in Jeeps. I have done this, and they quoted my $250 for a complete D30 (HP, non-disco, non ABS) and $350 for a C8.25, both with 4:10 gears. When I mentioned to him why I was contemplating this, he did indicate that I could see a drop in MPG from the increased RPMs.

Certainly, even when you factor in installation, this is far cheaper than installing new gears in the existing housings (pity this is the case), but then I got to looking at GoJeep's site, which has an interesting RPM chart at the bottom of his speedo drive gear change. This chart indicates that there's only a change of about 100RPM between 28-inch tires (P225/75/R15) to 30-inch tires. I am aware of the implications of increased tire width and mass, but am uncertain as to how much weight they carry in the equation, if you'll excuse the pun.

The last time I took the Jeep on the daily commute for a day or two, I checked the mileage and was getting around 19-20 (pretty good, IMHO, considering the oversized tires, skid plates, and rock rails), which is consistent with what I was getting back when it was stock and serving as my DD. Of course, that was before I got the A/C fixed, so I do need to try that again once I get the holes in the driver's side floor addressed.

Another observation I have recently made is that since I had the radiator replaced (7-8 years old, fins rotting away from age and salt), the truck feels a little peppier - maybe better flow through the cooling system has restored some accumulated parasitic loss?

As a result, I'm now of two minds - swap in 4.10 axles to get the hill climbing power back at the risk of annoyingly high highway RPMS and possibly decreased mileage, or stand pat on the stock 3.55s for now?

Thanks in advance for thoughts.

Rob
 
It sounds like you do have a dilemma on your hands. I too am running 30x9.5x15 on my '98. I've definitely noticed the same "hill problem" that you will be running into on your Moab trip. However, I have made up my mind that I'm not going to regear until after I get 32s on and then I will make my decision. I have heard of people running 32s without regearing as well. I wouldn't be able to stand the high revs on the highway. So that's what I'm going to do. Just my opinion. Hope it helps. (My XJ is also my DD)
 
There's no real reason for you to change, the small tire size difference isn't enough to be concerned about. However, if you wanted to go through the work and expense, you would very likely be happy with the increased performance you'd get from the lower gears.

It's a 15% difference, and you'd notice it both on road and offroad. Your tire size is a 7% increase over stock, so overall with the gear change you'd be 8% different than stock. I seriously doubt it would have much effect on your mileage, in one situation it might but in another it might help. It never hurts to keep the engine in it's good power band.
 
I too have been reading and searching threads for this answer and cannot figure this out. I am currently running on 31x10.50.15's, with stock gears of 3.55. I was wondering if there is a gear setup that works well with 31's and 33's. The reason I am asking is I am trying to figure out if I can get with 1 regear or not. My rig is a DD but, I still want it to perform well both on and off road. I was looking at a catalog and they had a chart that listed rpm's and gearing. I found my tire sizes and saw that 4.11's and 4.34's gave the best performance for the tire size. Can anyone help me out on this?

Thanks,
Silent
 
Silent said:
I too have been reading and searching threads for this answer and cannot figure this out. I am currently running on 31x10.50.15's, with stock gears of 3.55. I was wondering if there is a gear setup that works well with 31's and 33's. The reason I am asking is I am trying to figure out if I can get with 1 regear or not. My rig is a DD but, I still want it to perform well both on and off road. I was looking at a catalog and they had a chart that listed rpm's and gearing. I found my tire sizes and saw that 4.11's and 4.34's gave the best performance for the tire size. Can anyone help me out on this?

Thanks,
Silent
Click to expand...

Yeah, put the 4.56's in it and don't worry about it. Then hurry up and wear out your 31's so you can get the 33's.

You will actually be fine with the 4.56's. I had them with 32's and it was perfect, 31's aren't that different.
 
Rob Mayercik said:
Ok, I know this has been asked before, and I have read several recent threads on the matter as well as CRASH's missive in the stickies collection above. That being said:

I have been running 30x9.50x15s for a couple of years now on my '92, using the stock axle gearing (3.55). It's not been all that bad, but at times it has felt like the engine was laboring a bit on hills (I have on occasion run around town in "3" instead of "D" to keep the rpms up, but stick with "D" on the highways). The tach reads about 2K rpms at 60mph. Never really thought much about regearing (expensive).

Now that I'm starting the process of going over the vehicle to prep it for the run to Moab in October, I've been once again thinking about the gears. While I don't expect any issues out in the plains, it's the mountain climbs nearer the west end of the trip that concern me - whether I am going to run into trouble being slightly undergeared relative to stock. As I am already at 30s, I figured 4.10s would be the most logical choice, since the notion of going up an inch in suspension and maybe going to 31s when the 30s wear out has been on my mind since NACFest.

Obviously, I have done a little research on having my existing gears replaced (instant sticker shock, just add water). The guy I spoke to about it (local mechanic/speed shop) suggested I call a junkyard that specializes in Jeeps. I have done this, and they quoted my $250 for a complete D30 (HP, non-disco, non ABS) and $350 for a C8.25, both with 4:10 gears. When I mentioned to him why I was contemplating this, he did indicate that I could see a drop in MPG from the increased RPMs.

Certainly, even when you factor in installation, this is far cheaper than installing new gears in the existing housings (pity this is the case), but then I got to looking at GoJeep's site, which has an interesting RPM chart at the bottom of his speedo drive gear change. This chart indicates that there's only a change of about 100RPM between 28-inch tires (P225/75/R15) to 30-inch tires. I am aware of the implications of increased tire width and mass, but am uncertain as to how much weight they carry in the equation, if you'll excuse the pun.

The last time I took the Jeep on the daily commute for a day or two, I checked the mileage and was getting around 19-20 (pretty good, IMHO, considering the oversized tires, skid plates, and rock rails), which is consistent with what I was getting back when it was stock and serving as my DD. Of course, that was before I got the A/C fixed, so I do need to try that again once I get the holes in the driver's side floor addressed.

Another observation I have recently made is that since I had the radiator replaced (7-8 years old, fins rotting away from age and salt), the truck feels a little peppier - maybe better flow through the cooling system has restored some accumulated parasitic loss?

As a result, I'm now of two minds - swap in 4.10 axles to get the hill climbing power back at the risk of annoyingly high highway RPMS and possibly decreased mileage, or stand pat on the stock 3.55s for now?

Thanks in advance for thoughts.

Rob
Click to expand...

I'd have t osay stick with the gears you have. At least your not turning 35s with 3:55s. Its alot of work putting in new axles and you could run into things like replacing u joints, ball joints, brakes, and they could have pinion seal leaks, etc. you see where im going with this. Sure, the extra power would be fun, but on the flipside, thte money you save by keeping your axles/gears can go towards other stuff for youre rig. Just my opinion.
 
I've got 4.56's and like 'em. Mine's an '88 with the Renix 4.0 still (for now anyway) and, other than the motor being a worn out turd, it does fine both wheelin' and on the road. I wouldn't go any steeper in my rig.

That being said, the easiest way to figure out what i'd recommend you run for gears is to back figure from my setup to yours. You can easily do it by jumping a full ratio for each tire size change. For example, 35's and 4.56 are the same as 33's and 4.10...the same as 31's and 3.73's. Dropping a half tire size can be compensated for by ordering an odd ratio (the odd ratio between 4.10's and 4.56's is 4.27 - which by the way is an awesome ratio if it's made for your setup).

Since you're thinking of staying around 30 and 31 inches tall then I'd say you're not in bad shape where you are now for ratio. If you're sure you're going 31's then maybe 3.73's. With such a small ratio change though I'd have to think hard about the return considering the cost.

One of my rigs is currently on 4.88's and 37's and I get 65 @ 3000 rpm out of a 3 speed tranny. 4.88's and 37's are the same as 31's and 3.73's. Even with overdrive I don't know how much more gear I'd want if I intended on driving it to moab and as a daily.

Probably the most important thing to consider when picking gears is the overall driveability. I've picked the wrong ratio before in a driver and ended up getting 35mph be between first and second gear. That may not seem like a big deal but that means that in the 30 to 40 range the tranny is shifting up and down constantly and, if you're like me, you'll spend most of your time running in this range so it can get quite irritating.

It may be that your problem isn't the gears, it's the motor. Not enough power can cause you to have to shift down. The gears will help the motor get up to speed with less effort but the motor will have to spin faster to maintain that speed. Spinning faster may keep the motor in the power but it will also cost you more in fuel and put more stress on the engine.
 
Remember, the amount of fuel going into the engine is really based on the load put on the motor, not as much the RPM. If you are in overdrive, the motor has to work harder at a low RPM to maintain a certain speed (say going up a hill), so it'll be under more load and use more gas.

OP, I'm running 32's and 3.55's and its awful. Hearing you complain about having trouble going up hills is laughable to me. That being said, I just bit the bullet and ordered 4.56's yesterday to help my problem. If I were you and probably going from 30's to 31's, I'd just get those 4.10 axles and be done with it. They seem to be straight swaps, which you can probably do yourself or with a friend, instead of buying the gears for almost the same price, and paying a ton for a shop to install them. It seems like you do enough offroading to justify them.
 
sunburned said:
Remember, the amount of fuel going into the engine is really based on the load put on the motor, not as much the RPM. If you are in overdrive, the motor has to work harder at a low RPM to maintain a certain speed (say going up a hill), so it'll be under more load and use more gas.
Click to expand...

Sorry, in my tiredness I blended two sentences in my attempt to end an already excessively long blabber session...

FIrst thought: Spinning faster may keep the motor in the powerband on the big end but doing so will put more stress on the motor for extended lengths of time. 4 hour drives at 4000 rpm won't do you or your rig any good.

Second thought: if your gearing is to high (numerically low) then it may impact fuel economy in daily driving situations trying to get to speed

And yes, throttle position determins the amount of fuel injected not RPM or what gear the trans is currently in.
 
Exactly, which is why I geared down so much. I figure I can slow down a bit on the highway in exchange for getting up to speed much quicker and easier, plus not having to downshift on even the smallest hills. The roads I drive on the most are 50mph limits so I'm at very low RPM's in overdrive. When I get to a small incline, the Jeep just dies because it can't make it up the hill and has to downshift. It's pretty rediculous so I really needed to gear up. That and some mild towing and offroading made me go 4.56. I wish they made 4.30s for the D30 and 8.25 because that would be a perfect ratio.
 
I appreciate all the feedback, folks.

I have to admit, I am concerned that the junkyard axles come with seals/bearings/ujoints/brakes/maintenance history/etc. as unknowns, which could easily cut into the financial savings. Additionally, I'd likely have them installed at a shop(not much free time on my hands), which would further reduce those savings.

Being not that far off stock and still getting near-stock MPG on the highway is a significant argument for standing pat (especially when one considers how much fuel I'll have to expend to get from NJ to UT and back).

I'm hoping part of the laboring I have been experiencing has been a result of the old radiator - after I got it replaced, it seemed to feel like it revved up from idle a little easier when taking off from a stop, but I'm not 100% sure I'm not imagining it. Certainly I expect the temperature to be more consistent in the engine now, which may also help. Still, it's not uncommon for me to be in "D" and see the RPMs drop to around 1500 or so before it downshifts out of OD, so I worry at times that I'm lugging the engine (what's considered lugging on a 4.0, anyway?). It probably wouldn't be a bad idea to check out the TPS as well, as I don't think it's ever been changed while I owned the vehicle (and I suspect 225,000 miles is slightly beyond end-of-life for most sensors). Likewise, the O2 sensor is probably coming due (probably hasn't been changed since about 150K miles).

Probably at this point I should concentrate on the floorboard holes, then those two sensors, and then see how it drives. Seeing as the balljoints are probably original (unless they're on the knuckle assembly, in which case one side is probably original and the other has 100-150K on them) and the TREs are of unknown age/mileage, I want to do a pre-emptive strike on them as well, then drive the thing to work for a week to make sure everything is properly sorted well before the trip. Of course, I need to resolve the gearing issue before I do the balljoints, so don't be surprised if I revive this topic in a couple of weeks.

In the meantime, feel free to throw more logs on the fire.

Rob
 
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