SPD

SPD is a unit of speed used exclusively in Sonic Unleashed for the PlayStation 3 and Xbox 360. It is not present in the PlayStation 2 and Wii versions of the game.

Whenever Sonic passes a Star Post during day stages, the checkpoint will clock how fast Sonic is going and measure his velocity in SPD.

Value
The value of SPD in measurement is never elaborated in the game but given Sonic's top speed in gameplay is 179mph

One mph is equal to approximately 8SPD.