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Old 04-22-2019, 08:14 AM
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285/70/17 with 3.21 gears?

Will 285/70/17 work with a 2015 auto with 3.21 gears? Any input on how it will drive is appreciated. I’ve seen mixed opinions.

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Old 04-22-2019, 09:03 AM   #2
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Will 285/70/17 work with a 2015 auto with 3.21 gears? Any input on how it will drive is appreciated. Iíve seen mixed opinions.
It will "work", but it will be slower than it is with the stock tires on.
You will see mixed opinions because different people have different standards about how much slower is "acceptable".
We found that 3.73 gears and 32" tires were overgeared and too slow. But others will say that is perfect gearing.
I would just pick the size tire you want to run and gear for that size tire. Re-gearing a Jeep is not that hard or expensive. If you don't also upgrade to things like lockers or mechanical limited slips you can re-gear for about the cost of a set of tires.
But to each their own.

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Old 04-22-2019, 09:30 AM   #3
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3.21 is already undergeared for stock size tires.
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Old 04-22-2019, 10:07 AM   #4
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I throw in one of the varying opinions

I've got a set of 285/70/17 Duratracs on my auto transmission 2016 JK with 3.21 gears. I live in the country and most of my driving is on the highway. It's mostly flat prairie where I am.

I got my JK new with factory 31 inch tires on 16 inch rims. I drove with them for 2 years. I put the 285's on just over a year ago.

I've had no real driveability issues. I can do 0-60 MPH in 7.2 seconds. No problems merging.. I also have no problems passing on 2 way highways. I watch my gas mileage closely and installing the larger tires made no difference.

The worst I see is when I'm driving on the highway directly into a strong wind 20 MPH or higher. The transmission doesn't always shift to 5th. (But I can't remember if that happened with my stock tires).
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Old 04-22-2019, 10:15 AM   #5
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Will 285/70/17 work with a 2015 auto with 3.21 gears? Any input on how it will drive is appreciated. Iíve seen mixed opinions.
Other than the multi-color chart answer.... there are variables in the equation.

I have a '16 2dr soft top JK w/3.21's and wouldn't hesitate to throw on a set of 285's.
Elevation, usage, total weight, Tow, etc come into play...

I am not a fan of re-gearing but that's just me.
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Old 04-22-2019, 10:55 AM   #6
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I switched from 255/75-17 to 285/75/17 on my 15 JKU with 3.21 gears. Noticable drop in mpg, and on the highway is like 4th gear over 5th at any speed. Regearing will allow 5th to be used on the highway again and off highway should allow it to feel like stock depending on your driving habits.

I plan to regear mine and will probably go with 4.56 gears.
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Old 04-22-2019, 11:13 AM   #7
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I tried this size on my Rubicon with 4.10's and took them off after 2 days. Felt like my emergency brake was on and took longer to break. Replaced them with 34 x 10.5 KO2's (more like 33") and I was MUCH happier. Lighter, skinny tires with less rolling resistance.
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Old 04-22-2019, 12:56 PM   #8
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I tried this size on my Rubicon with 4.10's and took them off after 2 days. Felt like my emergency brake was on and took longer to break. Replaced them with 34 x 10.5 KO2's (more like 33") and I was MUCH happier. Lighter, skinny tires with less rolling resistance.
Are you referring to the 285/70/17 (32.7" x 11.2") the OP mentioned or the 285/75/17 (33.8" x 11.2") size the previous poster mentioned?
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Old 04-22-2019, 01:06 PM   #9
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This really depends on your expectations, and where yo drive it. Level ground, or mountain driving will make a big difference.

I run them size tires, but with a 4.10 Ratio.
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Old 04-22-2019, 04:05 PM   #10
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Couldn't we have started an oil thread instead..........if you already have 255/75-17 now, your talking about an inch difference. I would focus on getting a lighter C rated (or P vs LT) AT vs a E rated MT. Keep it lighter and less rolling resistance.
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Old 04-22-2019, 11:15 PM   #11
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Quote:
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Quote:
Originally Posted by EUG View Post
I tried this size on my Rubicon with 4.10's and took them off after 2 days. Felt like my emergency brake was on and took longer to break. Replaced them with 34 x 10.5 KO2's (more like 33") and I was MUCH happier. Lighter, skinny tires with less rolling resistance.


Are you referring to the 285/70/17 (32.7" x 11.2") the OP mentioned or the 285/75/17 (33.8" x 11.2") size the previous poster mentioned?
I was referring to the 285/70. The 34/10.5's felt more like stock. Skinnier and lighter than the 285/70. I had gone from stock, to the 285's to 34's all within a few days and it was very noticeable. The 285's actually made me check as I drove off to see if Discount Tire put the e-brake on when they pulled the Jeep back out for me.
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Old 04-23-2019, 12:08 AM   #12
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I'm running stock 29" on 3.21 AT Unlimited and i wouldn't advice larger tires.
On manual transmission 2 doors maybe 30-31 will work.
Even with 29" i have to manually downshift sometimes otherwise the auto transmission keeps the jeep in fifth gear till 1100-1200 RPM - i wont be surprised if they have the same transmission software for all gear ratios.
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Old 04-23-2019, 05:47 AM   #13
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My 17 has 3.21s and 305/70/16 tires, or right at 33 inches. It downshifts on every hill, especially on the interstate.

I have a set of 4.56s gears, but I'm hesitant to install them for fear it will be even less driveable with 33s and 4.56s, until I wear them out and decide on going to 35s.
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Old 04-23-2019, 06:33 AM   #14
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@Dreadnought870 - 4.56 will be absolutely perfect with 33s. Why hesitate? With 35s, I would run 4.88.
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Old 04-23-2019, 07:56 AM   #15
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I have that set up on my 2017 JKU Sport. I noticed a little difference from the stock 255s, but not much. I went with C rated BFGs for a lighter weight. I drive a mix of highway and local roads everyday. Does just fine off-road in Florida. No issues running 70 on the highway. In fairness, I'm not a fast driver and have had Jeeps and large diesel trucks my whole life. So, I don't expect fast acceleration. As someone said earlier, it's only about an inch difference, just watch the weight.
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Old 04-23-2019, 08:00 AM   #16
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Just curious, is the 33" the height installed? I believe that's just the uninstalled height. Not sure if that affect gear choice or not. I run 285 75 17 which are a 34" tire but installed are alittle over 33" and I to am considering 4.56 gears
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Old 04-23-2019, 08:33 AM   #17
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I've measured my 285/70R17 Goodyear Duratracs at 32.4 inches installed and inflated to 32 PSI. (Straight edge across the tread, tape measure to the floor). Size wise.. The spare doesn't even come close to blocking the high mounted stop light.

I have no idea why some people have problems with them when using 3.21 gears.
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Old 04-23-2019, 08:39 AM   #18
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I've measured my 285/70R17 Goodyear Duratracs at 32.8 inches installed and inflated to 32 PSI. (Straight edge across the tread, tape measure to the floor). The spare doesn't even come close to blocking the high mounted stop light.

I have no idea why some people have problems with them when using 3.21 gears.
Because some people expect more acceleration and power at the wheels than others do. Some people have hills where they live, or they like to drive fast, or they like to carry heavy stuff, or they tow heavy stuff. Or some people just don't know how much better it can be.
But to each their own.
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Old 04-23-2019, 09:15 AM   #19
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I'm not questioning the advantages of lower gearing. But the original question was about using 285/70R17's with 3.21 gears.

It seems that some see little difference from the stock tire and others can't manage to make it up a hill. I was just observing that it's strange that there would be such a difference.

Just a note.. I edited my post above on tire height. 32.8" is the unmounted height. Mounted the tire is 32.4 inches.
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Old 04-23-2019, 09:19 AM   #20
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Find someone who has a properly geared Jeep with the size tire you want to run. The difference will be significant. Or someone who has a stock Rubicon with stock tires and 4.10 will give you a good comparison.
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Old 04-23-2019, 09:28 AM   #21
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I was referring to the 285/70. The 34/10.5's felt more like stock. Skinnier and lighter than the 285/70. I had gone from stock, to the 285's to 34's all within a few days and it was very noticeable. The 285's actually made me check as I drove off to see if Discount Tire put the e-brake on when they pulled the Jeep back out for me.
Given the similarities in tire height, I'm guessing what is missing from the discussion (as @Nottalot alluded to) is load rating/weight.

You don't say which tire you are on, but using BFGs as an example:

According to BFG their KO2s in 285/70/17 in an "C" load rating weigh 50.33lbs or 51.37lbs, depending on which SKU you have.

Their KO2s in 34x10.5 weigh 54.85lbs each.

The KOs in 285/70/17 in an "E" load rating weigh 57.87lbs each.

If you were coming from KO2s in 285/70/17 in an "E" load rating, then each 34x10.5 tire was about 3 pounds lighter. But if you were coming from KO2s in the same size, but in a "C" load rating then the 34x10.5 would actually be 3.5 to 4.5lbs heavier.
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Old 04-23-2019, 09:35 AM   #22
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To the OP's question, I have 285/70/17 in a C load rating and an automatic transmission, but 4.10 gears. I used a programmer to recalibrate for the tires and I find the combo just fine, but I would be skeptical of 3.21 gears.
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Old 04-23-2019, 09:42 AM   #23
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@Dreadnought870 - 4.56 will be absolutely perfect with 33s. Why hesitate? With 35s, I would run 4.88.


I agree. Iím actually running 4.88s on 34s and itís great, actually hoping I donít lose much when I jump to 35s.


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Old 04-23-2019, 09:50 AM   #24
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I'm not questioning the advantages of lower gearing. But the original question was about using 285/70R17's with 3.21 gears.

It seems that some see little difference from the stock tire and others can't manage to make it up a hill. I was just observing that it's strange that there would be such a difference.

Just a note.. I edited my post above on tire height. 32.8" is the unmounted height. Mounted the tire is 32.4 inches.
As mentioned, I think the differences in opinion are down to different standards of what "acceptable performance" is, as well as differences in where and how different people drive. If you live in Florida and drive mostly steady state on flat land and don't carry much weight you may not have as big an issue with gearing that is too tall. Other factors would be things like do you off road it or not, and if so what type of off roading do you do.
As to performance, I remember years ago when I first rode a motorcycle. It was only a 400cc motorcycle, but it was the fastest thing I had ever ridden. I have long since learned that it was actually "slow".
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Old 04-23-2019, 09:54 AM   #25
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Given the similarities in tire height, I'm guessing what is missing from the discussion (as @Nottalot alluded to) is load rating/weight.

You don't say which tire you are on, but using BFGs as an example:

According to BFG their KO2s in 285/70/17 in an "C" load rating weigh 50.33lbs or 51.37lbs, depending on which SKU you have.

Their KO2s in 34x10.5 weigh 54.85lbs each.

The KOs in 285/70/17 in an "E" load rating weigh 57.87lbs each.

If you were coming from KO2s in 285/70/17 in an "E" load rating, then each 34x10.5 tire was about 3 pounds lighter. But if you were coming from KO2s in the same size, but in a "C" load rating then the 34x10.5 would actually be 3.5 to 4.5lbs heavier.
While tire weight is a factor, the differences in gearing that different diameter tires makes is the largest difference in performance.
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Old 04-23-2019, 12:41 PM   #26
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While tire weight is a factor, the differences in gearing that different diameter tires makes is the largest difference in performance.
I totally agree, but the post I was commenting on was saying that his taller 34x10.5 (probably 33.5" tires) had better performance than a shorter 285/70/17 (32.7" tire), which could only be explained by weight or rolling resistance.
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Old 04-23-2019, 02:25 PM   #27
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I totally agree, but the post I was commenting on was saying that his taller 34x10.5 (probably 33.5" tires) had better performance than a shorter 285/70/17 (32.7" tire), which could only be explained by weight or rolling resistance.
Fair enough. But those 34 x 10.5 BFG KO2's aren't really as tall as they are supposed to be. They are less than 33" as I recall.
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Old 04-23-2019, 03:03 PM   #28
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Here's another way of looking at it..

The 285/70R17 isn't much taller than the 285/75R17. The 285 is noticeably wider, but not noticeably taller. This is because of the lower aspect ratio (70 on the 285 vs 75 on the 255). Here's a photo I just took. I propped a 255/75R17 BFG KM2 up against the 285/70R17 Duratrac I have on my JK. (The KM2 came off a 2013 Rubicon).



The BFG isn't under load, but if it were, it would only lose about 1/2 inch. So, we're looking at gaining around 1/2 inch or perhaps 3/4 inch of tire diameter by installing 285/70R17 tires. That's a tire diameter increase of less than 2%. It's negligible. It shouldn't cause a major decrease in performance.

Now if someone were to tell me that changing the gearing to 4.10 would provide a performance increase, I would agree fully.

But again, the OP asked if 285/70R17 tires would work with 3.21 gears. My contention is that increasing the tire diameter by .5 to .75 inches (or under 2 percent) will have little impact on current performance with the existing gears. In my case, I never noticed any performance penalty. Yet others do.
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Old 04-23-2019, 03:13 PM   #29
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The 285/70R17 isn't much taller than the 285/75R17. The 285 is noticeably wider, but not much taller.
I think you mean 255/75R17
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Old 04-23-2019, 03:19 PM   #30
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Fair enough. But those 34 x 10.5 BFG KO2's aren't really as tall as they are supposed to be. They are less than 33" as I recall.
BFG claims the 34" x 10.5 are 33.5" (unmounted), so your number sounds about right when mounted. BFG claims the 285/70/17 is 32.8" unmounted, and it's certainly less mounted.

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