"Since the Earth is not a perfect sphere, there are three numbers to know when answering questions about the diameter of Earth. The rotation of the planet has slightly flattened it out, so it has a larger diameter at the equator than at the poles. The equatorial diameter of Earth is 12,756 km, its polar diameter is 12,713 km, and its average diameter, which is referred to in common usage, is 12,742 km. For our friends who are not using the metric system, that translates to 7,926 miles."
http://www.universetoday.com/15055/diameter-of-earth/
