You should always check/change the spark plugs if you don't know - plug wires wouldn't be a bad idea either. check the air filter, clean the MAF (with MAF cleaner spray, or q-tip and carb cleaner), run SeaFoam through the gas tank and upper intake (follow instructions on can).
2004 3.8 here, I get about 20-21 for 80% city, 20% highway driving with my dual exhaust, CAI, and performance 93 octane tune. So yours is about right. If I'm doing 100% highway I'm around 25-26 if I'm not leadfooting it (hahaha!)
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