Megametres (from the Greek words megas = big and metro = count/measure) are rarely seen in practical use, e.g. "5000 km" is much more common than "5 Mm". Unless dealing with astronomy, distances on Earth can't be more than 40,075 km (Equator length). The symbol (Mm) can also be confused with millimetres (mm) and care should be taken to use the correct capitalisation when using such symbols. Megametres are also occasionally found in science fiction.
The Earth's polar circumference is 39.94 Mm. (It was exactly 40 Mm from 1791 to 1875, but the definition of the metre was changed; see metre).