Bigger tires or reduce gears? - JKowners.com : Jeep Wrangler JK Forum
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post #1 of 22 Old 03-26-2017, 04:12 PM Thread Starter
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Bigger tires or reduce gears?

Its a silly question but I regeared my jeep to 4.88s a few months back and i currently have toyo MTs in 35s which for some reason only measure 33.75 despite having a lot of tread life. I drive a lot of highway and my jeep is a DD. I am struggling to go 70mph on the slightest uphill, and on interstates where I have to drive uphill like to Utah or california (i live in nevada), i have semis pass me because I cant go 75 without flooring it all the way. I know the jeep can do better than that. I just want to know people's opinions on cost and performance wise, should I regear down to maybe 4.56 or get 37s? I'm open to both and my jeep was built for everything. Thanks for the input.

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post #2 of 22 Old 03-26-2017, 04:17 PM
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Do you have a 3.6 or a 3.8 and is it a standard or automatic?

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post #3 of 22 Old 03-26-2017, 04:39 PM
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based on what your saying you are going the wrong way on gearing and tire size. If you are struggling with 35's you will really struggle with 37's and same with gears.
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post #4 of 22 Old 03-26-2017, 04:43 PM
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I wondering if he had an automatic if the shop that for his gears didn't reprogram his computer properly or if at all.

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post #5 of 22 Old 03-26-2017, 04:54 PM
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Rehearing helps you not need to downshift so much. If you're in a manual, downshift. If automatic and you're literally flooring it, presumably you are already at high rpm and then you just simply don't have enough horsepower to do what you want to do and an LS swap would make the most sense.
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post #6 of 22 Old 03-26-2017, 07:31 PM Thread Starter
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Sorry forgot to add that it is reprogrammed using the procal, and its the 3.6 with automatic. Its my second jeep but my first regeared one. i currently sit at 3k rpm at 70mph on a level highway. My friend who has a 1000 lbs heavier jeep with 37s on 4.56 gears says his is about 25-2600 rpm and doesnt seem to have the same issue, or maybe he just isnt complaining about it as much as me. I like to travel a lot and take my jeep with me but having to run on 3500 rpm on 4th gear for 2 hours straight uphill doesnt sit well with me.

i thought that by changing the tire size and gearing to "match" according to the tire-gear ratio chart would help me out a little, hence the moving up in tire size. but if gearing to a lower number would be more cost effective and even better on the engine than overall going bigger everything was my question.

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post #7 of 22 Old 03-26-2017, 07:47 PM
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You have to remember that those motors need higher rpm to get any performance out of it 3000 rpm at 70 mph is not an issue.

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post #8 of 22 Old 03-26-2017, 08:08 PM
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3k is pretty much where I sit with 5.13's with a 3.6 automatic on 37's at 70 mpg. Ive never really felt under powered but also I live in New Jersey so it's pretty flat.

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post #9 of 22 Old 03-26-2017, 10:18 PM
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Quote:
Originally Posted by Jkbroc View Post
3k is pretty much where I sit with 5.13's with a 3.6 automatic on 37's at 70 mpg. Ive never really felt under powered but also I live in New Jersey so it's pretty flat.

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^^^ Ditto. Except for the NJ part.

aiyadude, based on your last post, you still seem confused on the relationship between tire size and gear ratio, as well as what changing your gears is actually going to accomplish for you.

5.13s for example, are a "lower" gear ratio than 4.88s, even though the number is larger. The reasoning is that the pinion has to turn more times to make the ring gear turn once. That results in greater torque multiplication, higher RPM for a given ground speed, and thus a lower maximum speed. Sometimes you'll hear "low" gears referred to as "slow speed" gears.

Whatever gear ratio you may have, larger tires will raise the effective gear ratio, which means lower RPM for a given speed, higher maximum speed (if you actually have the horsepower to produce that speed), and less torque multiplication (poorer hill climbing ability).

The standard approach when one adds larger tires is to change to lower gears (numerically higher). This helps an auto trans find the right shift points and maintain the gear selected, keeps your engine operating where it was designed to operate, and because of the additional torque multiplication restores the "drivability" lost to the larger tires. Done right, the gear swap should put your engine RPM at any given speed, back in the same general vicinity it was from the factory (before you added the larger tires).

Your complaint seems to be with drivability, but also with what you believe to be too high an engine RPM. "Drivability" is very subjective and is impacted by many more factors than just tire size and gearing (weight of wheels, weight of tires, rolling resistance of tires whether MT or AT, viscosity of the gear oil in the differentials, ambient temperature, altitude, wind direction, external drag coefficient both inherent and that added by aftermarket accessories, etc., etc.).

IMO your 4.88s might be a little on the "low" side for your undersized "35s," which could give you a bit of an over-revving condition, but it should also give you better grunt and hill-climbing ability ( although you may have to slow down because of the engine RPMs). Many people run 4.88s with true 37s and are happy with the drivability and RPM. Others prefer 5.13s with 37s for a little extra grunt, but they will elevate the engine RPMs slightly. If you feel that your rig just doesn't have the oomph to get up the hill, then you need to opt for one or a combination of a few things: 1) more power, 2) smaller tires, 3) "lower" gears (numerically higher), 4) lighter weight, 5) decreased drag whether aerodynamically or internally (from fluids, lockout hubs, etc.). If you feel that your rig is already over-revving, then your solution is to go to higher gears (numerically lower) and/or larger tires, but I promise you your drivability in hills will suffer from that move.

It may be that you're expecting to maintain an unreasonable speed for the grades that are causing you problems. 3500 in 4th is definitely not excessive (on a grade) and doesn't sound like it's on much of a grade if it will maintain that. If it is on a significant grade, then your only problem is the noise; 3500 RPM in 4th gear for two hours will not hurt a thing as long as your trans fluid is staying cool. And if it's not, you need to slow down. If your horse is lathered, you're pushing him too hard. Slow down to a speed he can maintain. And if that speed is to slow to suit you, then you probably have the wrong horse to begin with.

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post #10 of 22 Old 03-26-2017, 10:37 PM
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post #11 of 22 Old 03-27-2017, 12:55 AM Thread Starter
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Dozer, good info. I'll admit for making the mistake of saying "lower" gears but I did mean lower number. Before my regear, I did as much research as I could with which gears to go for, and I settled with 4.88 for the belief that I can get some power and torque back that you mentioned in your post. And I do have that extra oomph but mostly noticeable on flat roads than hills, and on the rocks.
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post #12 of 22 Old 03-27-2017, 05:20 AM
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A big part of this problem is that you have one of the heaviest tires on the planet. Rotational mass plays a huge part in performance.
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post #13 of 22 Old 03-27-2017, 08:31 AM
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Quote:
Originally Posted by Dkjeep View Post
A big part of this problem is that you have one of the heaviest tires on the planet. Rotational mass plays a huge part in performance.
Him maybe switching to a Goodyear Duratrac or something similar would help​ I'm sure. I will say the arent the greatest rock tire though.

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post #14 of 22 Old 03-27-2017, 08:31 AM
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A big part of this problem is that you have one of the heaviest tires on the planet. Rotational mass plays a huge part in performance.
I'm not sure that's true in this case. Rotational mass takes energy to accelerate, I once calculated for a typical Jeep and for extremely heavy tires it might be 10-15% of the engine power when accelerating on flat ground.

But this post is talking about failing to accelerate up a hill, so the tires are just spinning at constant speed but not sucking power. All the engines power is going into wind damping plus overcoming gravity up the hill.
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post #15 of 22 Old 03-27-2017, 08:54 AM
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Gearing can probably help dial it in better, but tire size is probably the wrong direction (and weight is pretty much not relevant). The simple way to think about this is that your Jeep requires xx amount of energy to move its mass up a hill at a given speed. Think about a dyno chart and the power curve. The ratio of your transmission is limited to the 5 gears, so if you aren't making enough power at a certain RPM to maintain speed, it is going to downshift. In your case, it is probably going too far along the curve and spinning at an RPM that is higher than required.

Putting math to that would be much more complicated, but the point is that you are probably correct that you can spin a lower RPM than you are now when taking that same route.

4.88 is a lot of gear for that tire size. Far in excess of anything we'd ever recommend for a daily driver. We had zero issues with a 2014 JK on 35" Nitto's and 4.10 gears going up steep hills. Then we had one with 37s and 4.88 gears and it was not quite as ideal given the issue that you noted, where the RPM was higher than needed after the downshift. Aside from that, 4.88 felt like a bit much even with 37s for a Jeep driven on the highway.

The logic that things just get better as you go numerically higher in gear ratio is poor logic, especially with the 3.6 engine.

Lastly, your driveshaft is spinning at a constant RPM that exceeds what it was rated for. It can cause premature failure of the joints, pinion bearing, etc. You may not feel the vibes, but it is a great way to eventually break a transfer case into two pieces on the highway.
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post #16 of 22 Old 03-27-2017, 10:51 AM
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I'm running 37 sst's on 4.10s with the 3.6 JkU and it's not to bad . I hear some guys going 4.88s and some doing 5.13 on 37s I do a lot of highway driving at 80mph and when it drops all the way down it's at about 2700 on the rpm but it will kick up to 3k or even 4k for awhile but it cruises right along . I haven't made up my mind if I will have it regeard or not . I doubt I would do 5.13 though.

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post #17 of 22 Old 03-27-2017, 10:58 PM Thread Starter
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thank you everyone for the info. ill talk to my 4x4 shop and see what they recommend. maybe have a look at my jeep and see if theres anything sucking my power. as a city driver and constant 65 on the highway, I dont have any problems. I may just not be used to higher rpms but I will see down the road where it leads.

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post #18 of 22 Old 03-28-2017, 07:42 AM
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Another thing you may want to consider, see if any of your fellow Jeepers will let you drive their Jeep with the gear set and tires are you looking at. Since you are in Vegas, there is sure to be someone with your setup or what you are looking at around.

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post #19 of 22 Old 03-28-2017, 11:10 PM Thread Starter
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but that means I have to go find some friends! haha. all of the JK people I know have stock gears and the rest have yjs and tjs. hard to tell, but it is good advice.

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post #20 of 22 Old 03-29-2017, 06:39 AM
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Originally Posted by aiyadude View Post
but that means I have to go find some friends! haha. all of the JK people I know have stock gears and the rest have yjs and tjs. hard to tell, but it is good advice.
And you live in Vegas ? I can show you a shit ton of JKs in any gear, tire flavor you want.

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post #21 of 22 Old 03-29-2017, 10:00 PM
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Originally Posted by Dkjeep View Post
A big part of this problem is that you have one of the heaviest tires on the planet. Rotational mass plays a huge part in performance.
^^^^
This right here. I think the toyo mt's weigh in around 78-80lbs which is more that some 37's.

Sure my rig has seen the trail, but I wheel it like a soccer mom looking for a parking spot at Nordstroms.
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post #22 of 22 Old 03-30-2017, 08:09 AM
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Originally Posted by christensent View Post
I'm not sure that's true in this case. Rotational mass takes energy to accelerate, I once calculated for a typical Jeep and for extremely heavy tires it might be 10-15% of the engine power when accelerating on flat ground.

But this post is talking about failing to accelerate up a hill, so the tires are just spinning at constant speed but not sucking power. All the engines power is going into wind damping plus overcoming gravity up the hill.
Quote:
Originally Posted by Fusion 4x4 View Post
Gearing can probably help dial it in better, but tire size is probably the wrong direction (and weight is pretty much not relevant).
I don't think you guys realize how much power it takes to keep those tires that weight in excess of 100lbs per corner rolling at 70mph. Its not like it only takes hp to get them up to speed and then nothing else. Those things want to slow down period no matter if your rolling on flat ground or going up a slight grade. I challenge ANYONE to drive around on 33" tall regular truck tire and then switch to a toyo M/T of the same height and come back and tell me how it sucked the life out of it. I've been there and I've experienced it first hand. You cant honestly tell me that its only sucking 10-15% power when it feels like damn near 50%.

This is why racers use super light wheels, The Weld racing draglite didn't have massive popularity for decades because it was the best looking wheel out there. I've seen some rear wheel and tire combos on weekend racers that weren't even 25lbs per side. They don't do that for no reason.

Last edited by Dkjeep; 03-30-2017 at 08:16 AM.
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