Hey guys, so I currently have 235/75r15's on my 8 valve sidekick, and was wondering how much it would hurt highway drivin as well as gas mileage to go to something like a 30by9.5 or a 31by9.5. It's stock gearing, the vehicle is used as a DD and sees mild trail riding almost every weekend

You will probably loose low end power and acceleration will suffer. Hills will be a chore. You will find out fast how healthy your engine is. Differential gears will help/correct most problems.

The weight of the tire/ wheel assy will also factor in, so if you upsize consider weight and overall height of the tire/wheel .
 Alloy wheels could offset the tire weight some. And if you can compensate with gearing the losses will be minimal.

ive had 235's on my kick for most of the year and summer they are swapped for 31x10.5's

accel is crap. hills are crap. braking is long. you can tell the difference easily in how the truck drives.

BUT.

mine is on a sick motor which is down a cyl. back when i first got my 31's and had a healthier motor the differences werent that bad. all depends on the motor.

mines running stock 5spd and 5.12 gears.