It is true that partial fill-ups sort of wreak havoc on the whole idea of tracking average distance between fuel-ups, and average cost of fuel-ups. What would be interesting (though labor-intensive) would be to compile a list of tank sizes for various vehicles (which actually, individuals can easily do, to get an idea of what's happening). If I know, for instance, that I am averaging 32 MPG all-time, and my tank is 12.2 gallons, I know that my average distance between fuel-ups should be 32 * 12.2, or 390.4 miles. Ditto for average cost per fuel-up, based on the average price per gallon. I have tracked my own mileage so carefully, that when the 2-gallon gas light comes on (and I know I have about 60 miles effective range), I have felt comfortable traveling up to 40-45 miles. Talk about freaking out your passengers!