I've often wondered the same thing (the fuel consumtion part). In the past year or so, I've switched the cars I'm responsible for from a mileage based air and fuel filter service schedule to a 'fuel consumed' air and fuel filter service schedule. What prompted this is the following: consider these cars '97 Impreza, driven almost exclusively on the freeway, 85 miles/day; '99 Legacy, driven almost exclusively less than 2 miles at a time; '97 Legacy, mixed short trips, 5-10 miles each but only once a week or so; '02 Outback, mixed short trips, driven many times daily, with occassional freeway usage > 1/week, '02 Outback (yes, another one) driven < 2 miles to and from work daily, 30 mile round trip to school twice a week. Now these vehicles have 2 similar and one very different engine. All get very diffent fuel mileage. But ALL USE THE SAME AIR AND FUEL FILTERS.
I can't believe these filters could have gone through the same 'work' after the same mileage in these different vehicles under very different circumstances. I've settled on 900-1000 gallons of gas per air filter and 1900-2000 gallons of gas per fuel filter. It just seems to make more sense.
Dave