Well gas gauges aren't very acturet----------You need to fill it up, note odometer milage, drive it right around 100 mile fill up again and do calculation-repeat several times--- even go further-but keep a running record of miles driven and gallons used-I always only fill to auto shut off and don't top out ---------make it one of those graphie things and add notes to if long or short trips-------

You may have as many as 4 O2 sensors--also chech coolant temp sensor and Inlet Air sensor