I'd like to pose a question:
In a modern fuel injected, electronically controlled vehicle. If you take your foot off the gas pedal, the throttle plate closes, there is a higher restriction in the induction system, and therefore less air gets in the engine. The computer compensates by injecting less fuel, in order to maintain a target air-fuel ratio.
This would result in using less gas, yes? Be gentle with the gas pedal, and get more mileage right?
....so, when the air filter is a little bit clogged, isn't that a little bit like driving around with a block of wood under the gas pedal?
...so why would this result in a drop in fuel economy, if you are driving around with a higher restriction in the intake? (kinda like restricting your car to running at 95% throttle or whatever)
Seriously -- why does a clogged filter give LESS mileage??