A DIRTY AIR FILTER WILL NOT REDUCE FUEL ECONOMY! Sounds counter intuitive, but it's true! Here's why...
In a gasoline engine, the engine's output is controlled by choking it with the throttle body. It restricts the airflow to the engine at all times except when you put the gas pedal to the floor. Whatever the airflow happens to be, it is measured by the Air Flow Meter which then tells the engine how much fuel to inject. A dirty filter reduces airflow to the engine just like the throttle! The filter and the throttle restrictions combined results in the net airflow to the engine. If you have a clean filter, you'll end up using a slightly smaller throttle opening (gas pedal position) to accelerate at the same rate or maintain the same speed. If you have a dirty filter, you use a deeper throttle opening (gas pedal position). But in both cases, it is the net airflow that determines the amount of fuel you burn! The engine doesn't care what is restricting its breathing be it the throttle or the filter or both!
Now, a dirty filter will limit the maximum airflow you can get to your engine. It will reduce maximum horsepower and torque. So, it will make your car slower when you are really trying to go fast. With a really dirty filter, wide open throttle may be like 70% throttle. However, that WOT application will have exactly the same power and fuel burn as 70% throttle with a clean filter. It may take you 40% throttle to maintain 70mph in top gear instead of 25% throttle.But, again, that 40% throttle will have exactly the same output and fuel use as 25% with a clean filter. To get this bad you need to not change your air filter for 300,000 miles or something.ridiculous. Even then -- when driving "normally" around town or cruising down the freeway -- it'll have NO EFFECT on your fuel economy at all! A dirty filter is like putting a limit on how far you can depress your gas pedal! A slightly dirty filter may allow you 99% of your throttle. An unchanged filter with 100,000 miles on it may limit you to 90% throttle or something. But, it'll be no different from putting a brick under your gas pedal such that you cannot depress it fully. ZERO EFFECT on your fuel economy!
Don't believe me? Do a little experiment. Next time you do an oil change, don;t change the air filter (yet). Drive 10 miles down your favorite stretch of not so busy freeway at a fixed speed and measure your fuel economy with your trip computer. Now change the filter and drive down the same exact stretch at the same exact speed. You will notice ZERO difference in your mileage before and after.