Originally Posted by mpolli

...I thought there would be "pumping losses" as Bobk describes. However ForcedInduction got me thinking about the throttle plate. The throttle plate acts as a resistance in series with the filter. The engine cannot tell one from the other, it just sees a total resistance. The throttle opens until the resistance is such that the amount of air you "request" (with the gas pedal) goes into the engine. If the filter is more clogged, then the throttle will open more, but the total resistance WILL BE THE SAME. The engine will have no way of "knowing" the difference. There will be NO IMPROVEMENT in MPG. The only effect of a dirty filter would be a limit on maximum possible horsepower. (and possibly letting dirt in I suppose), but NO MPG improvement as advertised...