Maybe you sucked in a bird into the intake and the filter is now plugged up (Seen in a Ram Air Grand Am GT). Were you driving out side Ca and then got gas here in Ca (Noticed on my road trip out of state that my milage improved after buying gas outside the state of Ca and got worse after filling up again in Ca)? You should probably search for a fuel leak, or it posible that some Jack*** stole some gas from you. Did it do it just this last tank or has it been consistant since? Make sure nothing got unplugged from under the hood like a temp sensor or something. So there is no check engine light or anything like that. Have you checked for codes? Is it still under warrenty, I would take it to the dealer if it is. If the motor is running cold, or think it is, it will run richer.
Also, have you got any mods, like an open air filter or one of those resister things that trick the computer, all those will more and likly decrease milage as well. Was there any work done to the car just before you noticed the decrease in milage?
