DWINDLING Gas Mileage- 89 Jeep Cherokee 6cyl

Love my 89 Jeep Cherokee 6cyl auto 4wd (130,000). A real workhorse, takes lots of abuse, never lets me down.
EXCEPT the gas mileage which had avgd 15-18mpg (not terrific in the 1st place, but consistant) has dwindled over the past year to about 13mpg, no matter how carefully I drive. Of course this results in much pain at the pumps. Environmentally embarrassing also!

Please list the various things that could be the cause of my jeeps lost fuel economy-- thanx