I frequently see dirty air filter listed as a major factor (since it is listed in the first few items) in degrading fuel economy. Doesn't this just simply restricted the rate of air input and the ECU would then adjust the fuel flow to suit? I'm currently looking for the factors that has caused a sudden drop of ~25% (from 6.5km/l to 5km/l) in fuel economy in a 760 V6 and the above assertion puzzles me. In my case, as the oxygen sensor is making enough transitions between high and low value, the fuel mixture is probably ok. In my way of thinking this means the air filter and the air way is no longer a major factor. The poor fuel economy is caused by un-burnable fuel going through the system due to: a) Weak spark; and b) Fuel spray pattern or fuel droplets size. Any idea how to test for these problems? The other factors I can think of is drag due to things like brakes and fuel leaks, which I have found and corrected. Can anybody add something else?