We need a little more detail. Was the mileage drop sudden or gradual? Are you in an area that experiences real winter? Lower temperatures will lower the tire pressure along with other negative effects. Any recent maintenance done on the car? Change in gas station?
Check your coolant level too, a car with low coolant may take longer to warm up to normal operating temperature, when it finally does, it may then overheat causing the fan to kick in sooner and more often.