All About Shuttlecock Speed
In badminton terms, speed refers to how far a shuttlecock will travel when struck with a standard force - further being considered faster.
Different playing conditions can cause the same shuttlecock to have a different speed. The variables to consider from where you play are the differences of air resistance related to altitude, humidity and temperature. Because of weight differences and other factors, the shuttles come in varying speeds.
Fast or slow does not necessarily represent the quality of the shuttlecock.
The table below is the 3 most common system in use right now:
The smaller the number, the lighter the shuttlecock, the slower it flys. The shuttles were traditionally divided into 48, 49, 50, 51, 52 according to the weight of the shuttles. 49 weighs 4.9g, 50 means 5.0g, 51 means 5.1g and so on. Such rating was inaccurate and therefore was difficult to classify. It was changed to use Speed 75, 76, 77, 78, 79, 80 instead.
There is a 30cm distance between each speed. Shuttle speeds are ideal for different parts of the world. For example, shuttles with Speed 76 are used in China during the summer and 77 during the winter months. Shuttles with Speed 75 are good in Thailand. Speed 76, 77 in Singapore, Hong Kong and Malaysia. Speed 77, 78 in the U.S.A. Speed 78, 79 in Finland, Canada, Korea and Japan. Speed 79/80 in Australia in winter. Speed 73-74 would only be played in high altitude area such as Yunnan Province in China and Colorado Springs in U.S.A. or high temperature area such as Johannesburg in South Africa.
(Information source: http://www.shuttlecock.com/resources/shuttlecock-speed/)