first you need to have the computer scanned on it to see if it has any trouble codes on it ,then i would suggest you might want to do a really good tune up on it,usually when the plugs get a lot of miles on them this is where your gas mileage goes to ,if it wont burn the gas it just waste it,that would explain the drop in mileage,id try doing a real good tune up,that should bring the mileage way back up on it,those cars usually get terrific mileage,when it drops though you know something is wrong with it,good luck with it.