Keep your air filter clean. According to our tests, driving with a dirty air filter in modern engines doesn't have a significant impact on fuel economy, as it did with older engines. While fuel economy didn't change, however, power output did. Both cars accelerated much more slowly with a dirty air cleaner. We drove both vehicles with their air cleaners restricted and found little difference in gas mileage with either engine. That's because modern engines use computers to precisely control the air/fuel ratio, depending on the amount of air coming in through the filter. Reducing airflow, therefore, caused the engines to automatically reduce the amount of fuel being used.

I checked the link for tips. I'm not sure I follow their logic. First they say dirty air filters don't affect FE, that it just makes the car more sluggish. Then they say it simply causes the engine to reduce the amount of fuel being used.

I'm not sure these guys are mechanical engineers but it seems like what they are really saying is if one whould clog up their air filter then your auto's computer will decrease the amount of fuel to the engine and increase your FE? I know that's not what they meant, but it sounds like they have conflicting statements?