Most of the replies to your question so far have been suggestions of problems with the engine, or its state of tune, all of which are valid, and good things to check.

Only Bob W. mentioned a different possibility, that of a faulty odometer, however his experience was the opposite of yours---his odometer was recording miles faster than they actually happened, giving the appearance of excellent gas mileage. More likely in your situation is that your odometer either reads consistently low (sedans and wagons have a different instrument cluster--it could be a previous owner replaced the one in your car with an incorrect unit) or, perhaps more likely, is intermittent. When it's working, it reads reasonably accurately, but sometimes (more likely in colder weather) it will take occasional time off from the job, and unless you happen to look closely at it while it is out, and the car is moving at speed, you are unlikly to notice it. If this is the case, you will tear your hair out trying to find and fix an engine problem.