[Old Tele man]

...be sure to alternate DIRECTIONS when doing each coast-down test, and do so as soon as possible, so that wind speed and wind direction haven't changed, as they can do at morning and late afternoon.

...the idea is to do opposite-direction tests so that what you gain/loose going oneway, you loose/gain going the other way, so that the 'atmospherics' and 'road-grade' changes will effectively "cancel" each other out to yield a better (less influenced) answer.