You don't have a check engine light? With mileage that bad, something has to be seriously wrong. And something that wrong should turn on the check engine light.
Random thoughts -
- Do you have a transmission prolem? If you are driving a manual, is it possible your clutch is slipping? If an automatic, is it shifting properly? How's your fluid look?
- Is it possible you have brakes that are dragging? If so, you should be able to tell after a short drive. If present, a dragging brake should generate a lot of heat and possibly a real stink.
- Any chance you have a gas line, or fuel tank leak?
- A plugged exhaust system (plugged converter or muffler or a collasped exhaust pipe) can up the back pressure and wreck fuel economy - except I think this should turn on the check engine light.
- You probably should take it to a professional with the correct equipment.
Ed