This has been mentioned before, but let me recap a couple of factors:

(1) If you installed larger tires on the jeep, but didn't change your speedometer gearing or calcualtion, your speedometer is going to still be calculating miles based on the turns of your axles. However, those axles are now turning larger tires, which are covering a greater distance with each revolution than the old tires were. For example, if the original tires were 27", and you now have 31' tires, that's an increase of about 15%. So, if your odometer says you traveled 100 miles since your last fill-up, you have actually traveled about 115 miles. So, if your mpg calcualation comes out to 10.0 mpg, it would really be 11.5 mpg.

(2) Four cylinder engines are very inefficient if run at lower-than-designed rpm's. If you didn't change your differential gearing when you installed the larger tires, you re running that 4-cylinder engine at an rpm that is about 15% lower that the engineers designed it for. That means it will get poorer gas mileage.

(3) I think a figure of 18-20 mpg is pretty unrealistic for your Jeep. Even with the proper change in differential gears, and a speedometer gearing change, you are driving a fairly boxy Jeep that is now sticking up in the air higher than OEM, and thus catching more wind drag -- that will lower your mpg considerably.