I get about 10-15 mpg in my jeep. It has a 3 inch lift with 31's. I am wondering if there is any suggestions on how to increase the mpg.

When you changed from 235/15R75's to the 31" tire, did you change your axle ratios or have your speedo calibrated? Those are the first things that come to mind. Not only is your speedo probably off, showing less miles per tank of gas, but the engine may be working harder and outside the power band.

Changing the air filter can help, a simple tune up. Driving style affects gas mileage. Hammering the skinny pedal will hurt gas mileage.

i have had the speedo checked and i have a k&n air filter. I have been thinking about the new exhaust and am probably going to do that. If there is anything else that anyone can think of please let me know.

Certainly, these changes will increase your horsepower a bit. How much mileage improvement it yields depends on how you drive.

As the others have suggested a good "tune up" will should help. Also check your O2 Sensor.

I use my Garmin GPS iQue3600 to check MPH and distance against the speedometer.
Aren't those 31s a little tall for stock gears? Try some new plugs and air filter first. Clean the throttle body and plate too.

i have had the speedo checked and i have a k&n air filter. I have been thinking about the new exhaust and am probably going to do that. If there is anything else that anyone can think of please let me know.

How did you check your speedo? Its certainly off after putting the 31s on if you havent changed the speedo gear or diff gears. Youve probably killed your MPG when you put larger, heavier tires on, especially if you have any hills to climb, and increased the frontal area of the Jeep by lifting it.
You can waste (I mean invest)your money on all the performance add ons you want but the most improvement will be gained by simply driving like theres an egg under your right foot. For the life of me, I cant figure out why people hate hearing that, I mean it cost $0 to drive a little less aggressive and get better MPG. I would think thats better than spending hundreds.