Pilots are taught to take off and land into the wind, and avoid landing or departing with a tailwind. There is a reason: The performance penalty of a tailwind is much greater than the benefit of a headwind. How big a penalty? Go to your POH and calculate it. The most common figure is to add 10 percent to the takeoff or landing roll for every two knots of tailwind up to 10 knots. The specific penalty will vary based on a number of factors like runway surface, density altitude, and gross weight. Somewhere down in the fine print, you may see an additional penalty for runway slope. But often runway slope is neglected, because most runways are level.