Your mileage sounds just fine for the driving you do.
I average 21-23mpg..Didn't reset my average for about 2-3k miles last time and it was right under 22mpg average.
Now if it is just a highway trip and I reset my average mileage I obviously do better.
Easter weekend did a 300 mile trip..all interstate (290 miles of it at least) and I reset my average mileage when I left..
Had the cruise set at 78mph...averaged 29mpg on that trip...havent reset it since then and my average is right down around 23mpg now with all the city driving, spirited driving etc..
TSP 224/224 cam - Livernois Stage 1 LS6 Heads - JBA Headers - OBX Catback - LS7 Clutch - Lous Short Stick - TSP Tune - K&N Intake - 18X9 & 18X10 ROH Drift - 5000K HID - 15% Tint