He is faster than the speed of sound, there is no exact number. Hyper Sonic is when Sonic collects all 7 super emeralds, the hair change is a result of him going hyper. Being super drains him and he needs rings to stay super.
(sorry if I sent this one twice, I thought it didn't go through) Sonic can go faster than the speed of sound, but how fast is super sonic? What's hyper sonic, and why did super sonic's hair change. Why doesn't sonic like to BE super sonic often?