My stock 2004 gets 17 mpg around town and 21.5 on the highway. I average 24 on a long trip to the beach and 22 on the way back.
How many miles you got? I would suggest checking the air filter and spark plugs. You change your oil and oil filter, I recommend using 5W20 thibker oils have be known to leave build-up on the return ports. Some fuel injector cleaner probably wouldn't hurt.