[mwebb]

Frank Lee has the most accurate answer here .
the correct answer is....
NONE OF THE ABOVE , CHOICE C ,KEEP YOUR ORIGINAL AIR FILTER AND AIR DUCTING AND AIR BOX FOR BEST FUEL ECONOMY .

despite the huge sums spent to falsely claim and imply the opposite
k & n Air Filters DO NOT IMPROVE Fuel Economy
and they do promote early engine death due to influx of dust and dirt and other assorted doo doo that most other automotive air filters prevent from entering the engine .

the EPA and IATn have done extensive testing showing that there is no Fuel Economy gains to be had by replacing a dirty air filter with a clean or no airfilter - stock air boxes will most likely provide better flow than anything available after market so do not waste your funds on the crap sold by the scam artists , here is a way to check if your aftermarket filter actually is flowing as well as or better than "stock" http://autospeed.com.au/cms/A_111109/article.htmlauto speed article

in the summer warm air will provide a slight gain over cold air , heated air in winter will provide a larger gain over ambient temperature air

between the throttle body and the cylinder head ;
intake manifold runner length and diameter can be sized to maximize flow at the rpm used most often to improve fuel economy , Intake Runner and Peak Torque Calculator
use this or one of the many others out there for the values

Originally Posted by suspectnumber961

So with a 03 Ford Zetec engine...considering the just about even cold air/warm air tests...I'd be better off just removing the whole air intake tube and installing an OEM style K&N filter to reduce pumping losses from intake restriction?

Some claim mpg gains from the K&N OEM style and less tubing for the air to run thru can't hurt....would be drawing warmer air from under the hood though....