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I don't think you fully understand how unreliable your buddy's info is (no offense). Mileage is effected by tons of variables that change by the second while driving. Commuting back and forth to work each day does not return the same mileage every fill up. I'm sure your buddy didn't take any consideration to these variables when testing his new air filter. Now, consider the testing done by MetroMPG. He controlled as many variables as he could to get consistent data. Sounds a bit better, no?
If that doesn't click, consider this. When does an engine need the most airflow? When it is generating peak horsepower. At any other point in operation, it requires less air. So, by installing a less restrictive air filter, you are simply increasing the amount of horsepower you can pull through the filter. Considering a Ranger only needs ~25 horsepower to go down the highway, a stock filter is going to have absolutely no measurable restriction at 1/4 (or much less depending on the engine) of its total horsepower generation.
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