I don’t really pay attention. Fuel economy is not my first concern.
I generally add gasoline $5 at a time as the mass of a full tank of fuel effects the car’s driving dynamics. negatively.
On a long trip I fill up. I can get from Atlanta to Gainesville FL on my 13 gallon tank. (This is with performance modifications and a heavy right foot). Ignition timing is manually set for near maximum power, not fuel economy. However the car has been on a mild mass reduction program.
Full synthetic motor oil improves fuel economy notably. So do the rigid and slightly overinflated tires.