As Joe Padavano says, there are a number of items that can cause this type of mileage drop off. For a few of the culprits, it might be the engine coolant sensor, dirty air filter, mass air flow sensor or something in the air stream, leaky injector(s), bad spark plug(s), bad spark plug wire, intermittent bad coil, bad fuel pressure regulator on the rail, ambient air temp sensor, and the list goes on. Depending on the severity of the problem, the check engine light might not come on.