Well, did get 18mph. Nice job on the lift!
You may find you have lost some mpg from the higher profile and front impression to wind risistance. Not much loss though, but no longer 18mpg. Now if you find, after the new tires, that you still get 18, you, my friend, is very lucky.
I once had 21mpg on my 2000 XJ, then lifted 4", added 31s from around 27" or 28" stockers, removed all plastic off the front bumper to improve approuch angle, mounted Hi-lift and KCs on front bumper and havent changed gearing yet. I got down to around 13-15mpg until I bored out the throttle body and and did some sensor tweaking as per
http://www.go.jeep-xj.info/ and
http://www.angelfire.com/my/fan/CTS.html and
http://www.angelfire.com/my/fan/IAT_sensor.html , and have improved only to 18mpg. 18 is better than 13, check out the links, if you are so inclined, they may help you too. Since you have a '90, this also may help you...
http://www.angelfire.com/my/fan/MAP_adjuster.html , I haven't tried this yet since I have a 2000 XJ and mine differs a bit to this tip, but may be good for you. I hope you still get 18mpg, if not, I hope these links steer you to good help to recover the lost mpg's. Good luck, and good job so far!