[redpoint5]

The meaning of my first post is that conducting a non-controlled, not scientific test, and trying to derive scientifically valid conclusions is impossible.

If your friend truly cared about the hypothesis "cranked on the defroster on high temp...results in improved MPG", he would run the experiment and control for as many variables as possible. You don't need full tanks to run the test, just a reasonably accurate gauge to measure fuel consumed, or trip MPG.