Try filling it up once, drive it 200 miles, fill it again.
Miles driven/gallons used=MPG

 

Yeah this is really better than driving down to half a tank. And you need to do it multiple times to average out what mpgs you are getting.

 

first my 1996 cherokee only averages 10-12mpg. if i find myself working my second job more than once every other week(pizza delivery) it will easily shoot down 8-9, thats because i drive like a jerk and let it run for hours on those days. if your check engine light is on your computer reverts to a (safe) fuel trim that is usually on the rich side.(mpg sucker #1) secondly every time you reset your faults it can take up to 50 warm-up cycles in order to relearn the ideal fuel trim(mpg Sucker #2). lastly 3k is a little high to be getting up to in city traffic. everyone thinks they drive with a soft pedal but we really dont for instance drive with a grandma who's engine probably never see's 1800. bigger tires create more resistance if you want full benefit out them set them to damn near peak psi (44 on most but LT tires can handle 80). however that will adversely affect traction. and last thing to remember is these motors were designed in the 60's originally created as tractor motors. they were not designed to be fuel sipping snobs. that is all. but let us know what works.