I have a 09 JK Unlimited 4 door I'm ready to start modifying.
I want to replace the 32X10.50's with 35X12.50's. How much gas
mileage and smooth ride will I be sacrificing?

 

Well, depends... I have 35x12.50 17's on mine, and with a rock Krawler 3.5" lift and bilstein 5100's the ride is actually smoother than stock...(it's an unlimited rubicon) also I have a superchips flashpaq and coming back from Moab last week I had a best tank average of 21.5 mpg, overall trip hwy mileage averaged around 19 mpg, so really depends on how u drive it...

 

Im running the 35 inch treadwrights. i am averaging 13 mpg. most of my driving is to and from work only onthe highway for about 5 miles. if i wanted a fuel efficient vehicle i wouldnt have bought a jeep.

 

Wow that is amazing. Do you attribute that to the superchips flashpaq?

 

holy cow, 21.5? really?? i suppose if you drive conservatively and run a tire with less rolling resistance that anything is possible. when i was running 35" mtr's with a flashpaq, i could only average about 14 mpg. but then, i tend to drive fast on the highway and i'm sure that doesn't help anything. i will say that now that i'm running 37" falken wildpeak at's, i'm now running about 14 mpg again and this is as opposed to the 11 mpg's i was getting when i was running 37" toyo mt's. less rolling resistance = better mpg.

 

the wife and I both find that the ride is better, don't even notice a a lot of the potholes in the road that we did before.

 

I have Nitto trail grappler 35 x 12.5 on 18's and they ride super smooth. 2" mopar lift with slight rub up front. About 13.5 mpg

 

I am running 35 KM2s and i average 16 with 4.88 gears. The ride will definitely be rougher than stock, but not horrible

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Ya, somethings not right...I dont think I got 21.5mpg stock. I was getting about 14mpg with stock BFG M/T and switched to 35 Duratracs (AT) and am getting about the same. I also have a superchips, 2.5 coil lift, aftermarket (heavy) front and rear bumpers...no way your getting 21mpg with 35s (assuming stock gears)

 

Mine rides better with 35's and 3.5" lift than with stock and 32's. With SC Flashpac gas milage runs about the same hovering around 14mpg combo of city and highway.