The PERIMETER of the equator cycle is about 40,000Km. The Greenwich cycle is a bit shorter as the earth is a bit an ellipsoid. About 37,000Km by Google Maps. The distance on the envelope between ANY antipodes (=opposite points on the axis, I won't use "diameter" to avoid confusion) is HALF the perimeter (If you walk the whole perimeter long, you'd get to the same point, of course). The distance through Earth is
D=P/Pi=37,000Km/Pi=about11777Km