you could have a bad evap or purge solenoid, i think thats what its call, if a new gas cap dont fix it.
if you dont hit the highway much or at all, and drive around down all the time, like me, so mine drops as low as 12 mpg on the mpg average. even tho the instant it always up over 25 or higher. and im not hot rodding around town. pretty light on the gas myself.