Sounds like it's time to get serious. Two areas - the driver (known here as the nut holding the steering wheel) and the vehicle.
VEHICLE
Check for something dragging. You should be able to push it by hand on a level smooth surface. If not, check everything that rotates, especially brakes. You have three separate brake systems - front wheels, rear wheels, and parking brake (connected to the rear wheels).
Checking today Tire pressure is a biggie.
And going up today.
Check the engine from one end to the other. Timing chain, exhaust restriction, loose things that should be tight.
Transmission - manual or
automatic? If automatic, is the torque converter working correctly and is it shifting correctly?
Will have that checked today.
DRIVER
Are you timing traffic lights?
Yes Minimizing idling?
Never use a drive through. Practicing what we call DWB - Driving Without Brakes?
Yes, since age 17. I was taught that way. Studied the Master List of 65+ Mods in Ecomodding Central?
Yes Driving techniques can improve mileage by as much as 100%.
Good luck and keep trying.
Jim