Have you hand calculated your MPG?
EVIC (the digital readout) is wrong on many vehicles. If you live in a cold climate and let your vehicle idle to warm up for a long time that effects your MPG. Thats lots of fuel burned and no miles traveled.
Also if you have no changed your plugs yet. Look into that. Every 30k miles ...
I Average 18MPG with a lift and 32" tires. I get 21+ HIGHWAY.
Reset your EVIC MPG readout and see what happens.
I Average 18MPG with a lift and 32" tires. I get 21+ HIGHWAY @~65mph, if your cruise speed is 75-80 your getting worse MPG.
You have a roof rack? Anything on the roof kills highway MPG. I removed my cross bars even.
You have the air deflector under the front bumper in place? I have mine installed.
My XK is 2wd, which is less weight and less drag/driveline loss.
I have a hypertech tuner.
Wheel and tire weight probably contribute I have 18x9's with P255/70r18 Bridgestone Dueler RHS's. A narrower wheel and tire combo would be better, but a less aggressive tread (like mine), and lighter sidewall (P is lighter than LT) are ideal for MPG.
Full synthetic oil. Pennzoil Platinum 5w20 atm with mobil one filter for longer OCI's
New NGK plugs.
Driver MOD: My wife's DD. She has a light foot.
WHen I got this vehicle @~90k with stock height, fortezas, and OLD plugs it averaged ~14mpg.
Each of those things likely add .5MPG or less, but all told it makes a difference.