The ST170 can be quite sensitive to spark plugs, i have always been informed the best plugs you can buy are the OEM Ford plugs. Mine does 220miles to a full tank which is not unusual. If your lambda is getting sluggish this can cause bad MPG, it will be the first lamdba sensor you will want to change, the rear one has no effect on engine performance. It just senses if the first lambda has failed.
I always thought a car had better performance but was less economical in the cold, denser colder air will cause the engine to add more fuel to compensate.