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285/70 vs 34/10.5

7K views 37 replies 11 participants last post by  Overthinker 
#1 ·
I just upgraded to 285 KO2's and I have stock Rubicon rims (17 inch). The tire shop told me I can try these out and move to the 34's if I want to. I like how the 285's look but I keep wondering about the 34s. I think the 34's might make more sense for me since I don't do any rock crawling or play in the mud. It rains a lot here (PNW) though. And the only off-roading I do is on the beach or in the mountains during the winter.

I've read hundreds of reviews and watched videos and still have a question regarding rolling resistance. On the highway home from downtown, there is a steep hill. With the stock Rubicon MT, I was able to keep up with traffic just fine and I "think" I was able to stay in 5th gear (auto). Not sure, since it wasn't a big deal and I never paid attention. Definitely was 4th gear or higher. But now, it is down shifting to 3rd gear. I have 4.10 gears.

Would 34/10.5 do better? I know there is less rolling resistance, but the 34's are 3 lbs heavier per wheel. Also, being 1 inch taller, wouldn't the jeep be less aerodynamic with the 34s? And while I'm asking, which would get better gas mileage? Everyone seems to say the 34s because they are skinnier, but it seems like people don't factor in the extra weight of the 34s and the extra height of the jeep (more wind resistance). People claim there is a mpg loss with a lift, and I don't see how this is any different.

The mpgs are not important, just curious. I'm more concerned with being in 3rd gear on a Hwy. Thanks for any input.
 
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#2 ·
I could be wrong, but I think 12 lbs is negligible. I do think the skinnier tires would be better for DD-ing. I have 35/12.5 KO2's that only sit at 33.75" (under load) so keep that in mind when you think of height, I doubt the 34's will be anywhere near 34" after installed. I also have a 3.5" lift which in actuality is closer to 4-4.5" and my mpg has sucked (could be my tire width). I am regularly seeing 10 mpg with about 10-15% highway driving (I try to avoid it to be honest). I also have a lead foot at the lights, I may try babying it after next fill up just to see how much of a difference ECO-type driving makes. I think I might have a bigger problem (either rear alignment, over-bent coils in the rear, incorrect ProCal module setting (trusted mechanic to set it) or a combination of the three). I think I will reset ProCal tomorrow to make sure it was set properly for my tires (I'll remeasure tire diameter first).
 
#4 ·
Wow, you're analyzing this even more than I did. :) Your questions are all over the place and I'm afraid my answers will follow suit. Feel free to ask any specifics I don't cover well enough...

From stock Rubicon wheels, I'm now rolling on 285/75/17's which are (on paper) 34 x 11.5, but in real life, 33" tall. I have AT, 4:10, also live in the PNW, and have steep hill climbs on many common routes.

You mention stock Rubicon rims. I had to check your post history to figure that you might have a JKUR, and if so I think your biggest issue around here will be to keep the drive wheel from spinning on wet pavement when going uphill. But, of course, this isn't a highway problem; state and federal highway specifications are better than that.

The 3 extra lbs per wheel over your current setup isn't going to be noticeable, and if you reprogram the new tire size via Procal or similar device, your shift points will square back up. I love my new tires and wheels with this gearing, and I added 17 lbs per wheel over stock Rubicon rims and tires. Whatever that figures out to in seat-of-the-pants weight, I simply like the ride. I found the 4:10 gearing with stock tires/wheels to wind up too high for the freeway, but I bought the vehicle always expecting to go to these 34's.

The Rubicon is still quite a handful on our steep, wet roads, and I'm glad it's not my daily driver. I don't want any less contact patch, but if you're more comfortable handling the Rubicon in the rain, I think the 34 pizza cutters will be awesome.

Aerodynamic differences from stock to 34x10.5 has got to be negligible. I predict your gas mileage will be about .4-.7 MPG lower than stock. Mine is .7-1.0 lower than stock: an acceptable trade-off to me.
 
#5 ·
Thx, this his helpful. Yeah, I'm wanting to make a good decision now so I can stop thinking about it. Lol. I was leaning towards keeping them but then 3rd gear on the highway happened. I'm wondering if the narrower/taller/heavier tires would be an improvement or would be worse. If I knew this swap would put me in 4th gear on that hill, I'd swap immediately. I don't want to switch to the 34s and then ask the shop to swap back again.
 
#7 ·
Anyone have any input for the difference between 275x70x17 vs 285x75x17 running helo rims
275/70-17 are going to be way small. like shorter than stock 255/75-17's. 285x75-17 are almost as tall as 35's. I have run both and would never recommend the smaller ones unless you have a box stock jeep are going to keep it that way.
 
#11 ·
no stock is fine.
 
#17 ·
my question to the OP is what brand is the 285 you are running? Most 285's are going to be heavier than the 34x10.50's. Are the 285's a load range D or E, or P285?

Trying to make the same decision as this is going to be a daily driver and family vacation car in the mountains.
 
#22 ·
mine on a postage scale weighed 52lbs.
 
#25 ·
Still deciding between these two K02s my self. For those running these and have measured true height and width, can you please post those measurements. Thanks, Lee.
 
#27 ·
34x10.5-17 ko2 are 32.25" tall when installed on the jeep with ~30psi.
 
#37 ·
I can tell you that the 34's don't protect the rim much at all! All five of my rims have significant rock rash. But they have done very well offroad on the rocks, in the mud and the beaches of OBX. Even on the large wet rocks I never had a problem. Anxious to see how they handle the snow, supposed to be a big snow season this time around!
 
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