To make the test more accurate and valid, you should have done your two fill ups there and two fill ups back (or however many, basically meaning starting off and ending with a full tank with both set ups), and calculated your mileage by dividing miles traveledby gallons used. Those trip computers are notorious for being off by as much as 3 miles per gallon, it shows only theaverage use,and doing you whole trip without a reset averaged in both setups

 

Sounds like you made a really scientific test and accurate conclusion...

 

Yep, I live on flat ground also, dang near sea level too. Oklahoma and Kansas do have good winds as 94rt10ohio stated and our trucks aren't the most aerodynamic things on the road.

Also consider this, did you use the exact same gas? (brand/octane/did it have ethanol)

 

I have a K&N drop in air filter too. and did any of you read the package its not made to increase the gas mileage. it says right at the bottom customers report that there were increases in gas mileage. or something along those lines. from what i was told it doesn't make any difference at normal highway speeds. that it only performs better when your driving the hell out of your truck. i dont know i should try doing some testing with mine and let you know what i come up with i just got one because it will last as long as i have my truck unless i wreck it cleaning it.