The Wachoskis are apparently sensitive to who fills the roles most closely connected to the title character of Speed (to be played by Emile Hirsch) and want to make sure they cast exactly the right persons. In the original series, it was Speed's father who created the Mach 5 car in which Speed Races, which the mother is crucial to the racing team.
Other roles yet to be filled include Speed's girlfriend Trixie, and Racer X, the villain of the piece.
Source: Reuters.