265/75r16 to 285/75r16 on a 3.21 JKU

#1
Hi-- I have a 2016 JKU auto 3.21 gears and currently have P265/75r16 Hankook ATM's on stock 16" wheels. They are about 39lbs. I'm thinking of moving to LT285/75r16 all terrains that will weigh about 57-60lbs for no other reason than I'd like slightly bigger tires. Does anyone know from experience what kind of hit I will take in MPG, power, etc.? The stock tires were p225/75r16's and other than the speedo being off I don't notice much power loss except rare times when I'm on the highway in the mountains. Unfortunately most 285/75r16's are heavy D or E range and 255/85r16's are just as heavy E range mud tires. Thanks!
 

Zeep

Adventurer
#2
Your biggest problem is in your gearing. The weight of the tires will be irrelevant, with a proper ratio. I don't know if your running auto or manual, but you should have a minimum of
3:73 manual, or 4:10 auto.
 
#3
Auto, and I’m not able to regear at this time. I guess my question is how badly the heavier larger diameter tires will affect things with the 3.21 gears.
 

Zeep

Adventurer
#4
You stated "for no other reason than I'd like slightly bigger tires". For the highway you'll probably be fine, off-road, you're going to fall on your face!
 
#5
using the size numbers, this figures out to less than a 4% difference. Assuming the actual rolling diameters of each tire is proportionally equivalent, you can expect less than a 4% decrease in performance from the larger size. I doubt you will be able to 'feel' this small change. However, the increase in weight is something you may feel when accelerating. Once you air down for Off road, the difference will most likely be negligible.
 
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Happy Joe

Apprentice Geezer
#6
You are wanting to go from a 29 inch (tire (approximately) to a 31 inch tire (approximately) with 3.21 gears... (not something that I would do, personally).
You can probably use the larger tires, expect: worse performance, poorer acceleration and increased stopping distance along with reduced fuel mileage; if it does not have one, a transmission cooler, or a higher rated one, may help keep the transmission temperature down, extending its life.
When switching tire sizes (on vehicles with low/moderate torque & horsepower, approximately 200 ft pounds and 200 horsepower); its mostly about leverage (tire diameter)/gearing; tire and wheel weight makes a difference in braking distance but relatively little in performance (unless the engine is terribly underpowered).
The load range of the tires affects sidewall stiffness; higher ratings leading to poor ride in vehicles carrying light loads. I go for load range C with a 3 ply sidewall whenever possible to try for the best compromise between sidewall strength and ride.
 
#7
It’s actually going from almost 32” tires to almost 33” tires but the rest holds true. What I’m hoping is to find someone running 285/75r16’s on a stock JKU with 3.21 gears to find out the real world difference. I think the issue will be more the weight and the stiff carcass than the diameter.
 
#8
I run 35 with 3.21's. Ideal no, but doable until I can regear. I haven't even changed the computer with a programmer for the different size tire yet. My onboard says I'm getting 15 mpg, but without the reflash of the computer, it can't calculate it correctly. I didn't buy the jeep for good gas mileage so I haven't done my own calculations.
 
#12
You will be fine going to the slightly larger tires. Offroad, you won't notice too much of a difference other than a little better ground clearance. Where you will notice a bigger difference is in highway/interstate driving. When going up hills at those speeds using cruise control, it will slow down by about 5mph before shifting into a lower gear. If this gets irritating, you could just turn off overdrive.

BTW, I have a 2008 JKU with 3.73 gears and auto. I have known others that have the 3.21 gear ratio and larger tires. I went from stock tires to 265 75r16 BF Goodrich AT KO's to 285 75r16 Goodyear Duratracs. This summer, I plan to go to 295 75r16 BF Goodrich AT KO2's. I would highly recommend the BFG AT KO series.
 
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