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Discussion Starter #1
Anybody running 285/70/17 tires with a 3.21 gears on a 2014 2dr JK with a 3.6 motor with and automatic. I am currently running 255/75/17's and want to go to a 33" tire. looking for people who have had or have this setup to chime in. did you notice much of a change in power, or MPG currently getting 19+ when I drive below 65 a little more if I draft a big rig. Buying tires next week and I have over analyzed this on my own I am a jeep newby had rig 5 months
thanks Woody
 

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3.21 is undergeared for anything over a 29 inch tire. I would strongly recommend regearing with any larger tire.
 

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I run similar sized tires (33x12.5R15) with 3.21 gears and IMO as long as you change shift points using a programmer (like an AEV ProCal or Flashcal F5) you'll be fine. It is still somewhat more sluggish than stock but works just fine for me as a daily driver, including driving 70mph+ on the highway.
 

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I'm running the same setup except I have a manual, it is fine on and off road. I keep up with daily traffic and the other jeeps on the trails.
Fuel economy is 13L/100k and I can't use 6th gear, if I ever find the money to re gear I will but really only to make the harder trails a little easier on the jeep.


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I run that size tire in the winter with the same 3.21 auto (2013 JKU) and it is a noticeable drop in performance.

To the point that my wife longs for the stock Sahara wheel/tire combo, and fully supports a regear.

In the spring when the Sahara rim/tires go back on she comments on the improvement.

Main thing I notice is that going up hills (in the Rocky mountains) that it will want to go up in 3rd or 4th rather than 4th or 5th.
 

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3.21s are about as useful as screen doors on a submarine. Only good for MPGs when cruising. I'm in the same boat (4drjk w/3.21s) and I just went down to 17" wheels fromn18". I'm on the fence about 295/70s or 285/70s. And I'm also about to step up to 4.10s since they run fairly cheap for front and rear. Atleast you have a better trans and more power with the 3.6 vrs my weak 3.8.
 

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Running 33x12.50x15 duratracs on 3.21 gears. Sure it's a little underpowered compared to the stock 29" tires but it works just fine. It's not a race car, it's a box on wheels lol. The high gears aren't really necessary unless you're rock crawling which most jeeps probably aren't doing. It's been very capable on and off road so far. I was getting about 18mpg with summer gas but with the winter gas I'm around 16mpg


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Discussion Starter #8
Thanks for the reply's My JK came stock with the 32" 255/75/17's on the Willy's wheels and the power is fine was looking for how much of a drop I would get to see if I could pull off 285/75/17's from what I see here 285/70/17's are as big as I can go. I already have an Eaton E-Locker in front so to re-gear I would have to replace locker as well. 2500+ to regear is not appealing at this time. If I change my mind I would go with a 44 in front. But even if I had to go back to the 255/75/17 tires I would still be happy with what it can do. I don't do tougher than 3+ rated trails so even the 32's can do that with lockers and an automatic. plus I have a winch and belly armor if I have to drag myself up.in the process of trussing front axles for extra protection with 33" tires
 

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I run that exact size in Cooper Discover ST MAXX. I have a 14 JKUS. Front and rear bumper with a winch. It is an auto with 3.21 gears. I have not noticed a significant drop in performance. I also pull a 12x6 enclosed trailer almost daily for work. When i am not pulling and not fighting the oklahoma wind i get 16-17 highway mpg. I have had no issues with the front 30 either. The c's are gusseted. No truss or sleeves. I am in the process of building a Currie RJ44 and adding lockers and re gearing however.


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I am running BFG KO2's 285/70 17s with 3.21 gearing. 2017 Jku Sahara with XD Rockstar 2 wheels. Maybe a rookie mistake, but all runs fine around town and on the highway.
 

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I did this math for another thread, but it's relevant here too.

Specs say that it's 663 revs per mile on the stock Silent Armor tires, and 638 on the 285 Duratracs. 25 tire revolutions....at 103.67 inches for the 285s vs. 98.96 inches for stock....That makes for 117-3/4" or less than 10' per mile difference.

Is that going to make a noticeable difference? Probably not much. Someone will inevitably post the Sacred Gear Ratio Chart, and that really doesn't mean much (as I'm turning a tad over 2,000rpm with 3.21s and 31.5" tires.). You're adding 1-1/2" to the diameter of the tire, it's not going to be a night-and-day difference.....The difference will be felt due to the increased rotating, unsprung weight.
 

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I did this math for another thread, but it's relevant here too.



Specs say that it's 663 revs per mile on the stock Silent Armor tires, and 638 on the 285 Duratracs. 25 tire revolutions....at 103.67 inches for the 285s vs. 98.96 inches for stock....That makes for 117-3/4" or less than 10' per mile difference.



Is that going to make a noticeable difference? Probably not much. Someone will inevitably post the Sacred Gear Ratio Chart, and that really doesn't mean much (as I'm turning a tad over 2,000rpm with 3.21s and 31.5" tires.). You're adding 1-1/2" to the diameter of the tire, it's not going to be a night-and-day difference.....The difference will be felt due to the increased rotating, unsprung weight.


Well said. To much math for me but spot on. Harder to get moving with added weight but once it is moving no worries.


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I did this math for another thread, but it's relevant here too.



Specs say that it's 663 revs per mile on the stock Silent Armor tires, and 638 on the 285 Duratracs. 25 tire revolutions....at 103.67 inches for the 285s vs. 98.96 inches for stock....That makes for 117-3/4" or less than 10' per mile difference.



Is that going to make a noticeable difference? Probably not much. Someone will inevitably post the Sacred Gear Ratio Chart, and that really doesn't mean much (as I'm turning a tad over 2,000rpm with 3.21s and 31.5" tires.). You're adding 1-1/2" to the diameter of the tire, it's not going to be a night-and-day difference.....The difference will be felt due to the increased rotating, unsprung weight.


Well said. To much math for me but spot on. Harder to get moving with added weight but once it is moving no worries.
Unfortunately, it's a little off. The 10' difference is just the difference in the distance of the 25 revolutions. See the below post for more math fun.
 

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Ok, this whole math thing is bugging me.

There are 5280’ per mile.

Silent Armor – 31.5” diameter, which should yield a 98.96” circumference, which is advertised to turn 663 RPM/mile….So 5280’ divided by 663 revolutions equals 95.57”….Hmmmm

Same math with the Duratracs…33” diameter, 103.67” circumference, 638 RPM/mile….Which gives us 99.31”.

So using that method, there’s a 3-3/4” difference in circumference, versus a 4-3/4” difference using the math to get the circumference from the radius.

And of course, a mile’s a mile no matter what tire you’re spinning (and that point was very unclear in my initial post).

So my Jeep’s calibrated to assume that in a mile, we’re traveling 5280’ with 663 tire revolutions. With the 285/70R17s, in 663 revolutions I’d be traveling nearly 5,487’, or 207 additional feet, which is a 3.9% difference. Going by the circumference method, the 285 travels 5,728’ in 663 revolutions, or an additional 448’, which is an 8.5% difference.

I have no idea why Goodyear’s numbers are so different, but maybe their RPM numbers are based on laden circumference. But I tend to think that's probably the more accurate number. That being the case, the speedometer would be off 2.73mph at 70mph (actual).
 

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I have been looking at upgrading to a 285/70/17 tire also with my 3.21 gears. Instead of worrying about the additional 1 - 1 1/2" of height, I have been looking at the "weight" of the individual tires. To me it seems the additional weight of the larger tire would play into the performance more. This is my opinion so more than likely it's wrong.

As a starting point, the base stock 255 Wrangler SRA tires are 36lb a piece.
These are weights of the 285/70/17 sizes:
- Wrangler duratrack - 53lb
- BF Goodrich T/A KO2 - 57lb
- General Grabber AT2 - 61lb
- TOYO Open Country AT2 - LT version 56lb , "P" Rated version 48lb

So as you can see in just that small sampling there is a big variance in individual tire weight. Personally, I am looking at putting on the TOYO AT2 in the "P" rated version. It will give me the height and traction I need without too much extra weight. Since I do not rock-crawl, the added sidewall protection and shoulder traction of the LT version tire is not needed. I read on another thread that with the 3.21 gears the 12lb added weight per tire is similar to having a 400 lb load in the cargo area at all times.
 

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Math... so confusing. The issue isn't just the distance... it is the percentage of change in the effective gear ratio. Just like going from 3.21 to 4.10 is a big difference...(but it's only .9!)... .9 of what? That is a 25% change in gear ration, and for sure you are going to notice it.

Starting with a 3.21 ratio and going up-size tires you are at a mathematical disadvantage, because the percentage change for you is going to be much greater. Going from stock to 35's for example is around a 7-8% change in circumference...

Of course your Jeep will turn the tires. It will lug the engine more. On steep terrain it will gear down more. In many cases it will eliminate 4Hi from being an option for trails and you will be in 4Lo a lot.

Does this 'matter' to you? Maybe, maybe not. Lots of people are AOK with the performance. Lots of people aren't. Math is on the side of re-gearing, but there is also the math of money... real trail rigging costs real dollars...
 

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Math... so confusing. The issue isn't just the distance... it is the percentage of change in the effective gear ratio. Just like going from 3.21 to 4.10 is a big difference...(but it's only .9!)... .9 of what? That is a 25% change in gear ration, and for sure you are going to notice it.

Starting with a 3.21 ratio and going up-size tires you are at a mathematical disadvantage, because the percentage change for you is going to be much greater. Going from stock to 35's for example is around a 7-8% change in circumference...

Of course your Jeep will turn the tires. It will lug the engine more. On steep terrain it will gear down more. In many cases it will eliminate 4Hi from being an option for trails and you will be in 4Lo a lot.

Does this 'matter' to you? Maybe, maybe not. Lots of people are AOK with the performance. Lots of people aren't. Math is on the side of re-gearing, but there is also the math of money... real trail rigging costs real dollars...


The effective change in final drive ratio going from a 31.5" tire to a 33" tire is nearly negligible.....you're getting an effective ratio of 3.09:1. That's just not enough difference to sweat. Of far more consequence is the increased weight and rolling resistance, and yes, deeper gearing would help overcome those things.

And I'll never use my JKU to do any serious off-roading, nor will most owners. The only things I'll notice with larger tires is slower acceleration, longer braking, and worse fuel economy.
 

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It is going to be a combination of the unsprung weight (and where that weight is located), and the change in circumference that will result in a change in how the Jeep feels / accelerates / hunts for gears.

FWIW, even a small change in weight is noticeable. With the same tires going to a lighter wheel on my RX-7 was noticeable. A 17 inch rim that weighs 17 lbs is very different than a 17 inch rim that weighs 25 lbs.
 

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It is going to be a combination of the unsprung weight (and where that weight is located), and the change in circumference that will result in a change in how the Jeep feels / accelerates / hunts for gears.

FWIW, even a small change in weight is noticeable. With the same tires going to a lighter wheel on my RX-7 was noticeable. A 17 inch rim that weighs 17 lbs is very different than a 17 inch rim that weighs 25 lbs.
True, and the larger tire has more of its weight further from the hub, which makes for a higher polar moment of inertia.

But there's a bunch of big differences between an RX-7 and a JKU, starting with the complete lack of low-end torque from a Wankel. On anything with a rotary, you've gotta spin the piss out of it to get it going. So yes, absolutely, a couple of pounds off of the wheels will make a dramatic difference. The RX is also much lighter, so 32 pounds off of the wheels will make more of a difference. You'd be hard pressed to feel an 8-pound-per-wheel difference on a JKU (with all else being equal).

Look, I'm not saying that 33 inch tires with 3:21s are optimal, but they're not going to be awful. In my case I'd be adding 13lbs. per corner, a tire that's 1-1/2" taller, and has 1-1/2" more tread on the road. The combination of the weight, the polar moment of inertia, the increased friction due to the larger contact patch, the reduction in effective final drive ratio, and the increased air resistance will all play into the degradation of performance.

Judging by the vast majority of those who report living with this combination, it isn't a big deal. Whenever I wear out the stock tires, I'll report back with my impressions.
 

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The unsprung weight change is going to reveal itself far more in your shock performance than acceleration... Going from the lighter-spec stock tires there are 35" options that are nearly twice the weight... That's a lot of rotational mass to keep from bouncing up and down on rough roads. I found running my 35's a little soft made the ride a lot nicer, and hey, you're already throwing away 3-4MPG, might as well dive right into H2 range...
 
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