A sidekick should get better than 22mpg. lil Suzy got 30 when she was new, She gets 23 now with a tired engine 31X12.50 boggers, lockers and 7" of lift. First place I would look is at the injector, or injectors depending on what year it is. If an injector goes bad it will not fully atomize the fuel and cause poor performance and bad mileage
Dave