I changed the air filter a few months ago in my truck to a Fram..yes i know fram isnt the best, but its all they had at the local autozone...I couldnt tell what the old filter was because it was so..well old...anyways, my fuel mileage went from 21-17mpg since i bought the new air filter. Could the air filter do this, or is there something else i need to look at?

 

a few months ago it was october and winter fuel may not have been at the pumps yet. your decrease in mileage could be for that reason...

 

winter fuel and the cold weather are probably the culprit...a more restrictive air filter can cause this but ive seen the fram filter and it doesnt look any more restrictive then the fleetgaurd....bhaf it, it will last u a few years and it flows more,cleaner air....the stock air box leaks tooo much.

 

The air filter deifinitely COULD do that. otherwise we would all have LTAF instead of BHAF.
The only way to tell is to switch out. I noticed a dif when I put an oil bath foam filter from the standard pleated paper. but I'm not ready to go to a Little Teeny Air Filter yet, no matter how good the foam is.