Well, first welcome to the forum. Second you posted this in the CJ forum, so you won't likely get much response since you own a TJ. But your speedo/odo are likely off due to the oversized tires, and that is giving you inaccurate readings to calculate your mpgs... Run with a GPS to establish if in fact the speedo is off. Probably is by about 5mph at 60... meaning if it says your going 60 you are really going 65. If this is the case then your mpgs are pretty close to average, being they are closer to 16mpg. The only thing really you can do is go back to a smaller tire/wheel combo which will weigh less and therefore get better mileage. But at that about 17-19 is max on the highway.