Bad fuel mileage

Just a partial list of things to check; timing, injector pulse (with a noid light), all the tune up stuff (plugs, coils, wires, cap/rotor), leak down test and/or compression test to look for excessive blow by, wear on torque converter or on clutch, dragging brakes, fuel pressure/filter and fuel regulator. Do you have a check engine light on? Transmission issues may not set codes, and things like late shifting or loss of O/D can really blow your gas mileage.

Yeah I have a check engine light on but haven’t had it diagnosed yet.

Any parts store can read it free of charge. The OBD code will be your biggest clue as to what's wrong with it.

I also have a dash reading which says just gas cap