What could cause a sudden drop in gas mileage?
If you mileage has dropped dramaticly in a short span of time there is one very probable answer. The thermostat to you engines cooling system may have puller through into the engine itself, or simply be in need of a replacement. With no thermostat the coolant is constantly circulating the engine. It is supposed to only do this at a higher temperature but if the engine is constantly cold it will be running a rich mixture of fuel, causeing the mileage to be extremley poor. This thermostat should not be an expensive fix and i would suggest looking into the situation as I have seen it before.
