My 1999 Mitsubishi Galant gets bad gas mileage?

Its rated at 18-22 mpg in the city. I gave it a tune up including k&n air filter, plugs, wires, oil changes, new tires and keep them full, and fuel system treatments. There is no fuel filter unless its built into the tank. The injectors also tick pretty loud, Ive seen these cars and they're always quiet? Any suggestions appreciated:) p.s. i also drive humanely most of the time, not off and on the gas etc. lol