A MD80's autothrottle system failed during takeoff because of a number two air data computer fault. The First Officer's airspeed was in error and the Captain rotated 10 KTS early according to the First Officer's reading.

Narrative

Takeoff Runway 18L. Captain flying. First Officer calls out '80 KTS.' Auto throttles disconnected shortly after and EPR Chevrons began to roll towards 1.0. I advised Captain. Captain rotated about 10 KTS early (on my airspeed indicator). When I saw V1 I called it out with a tone that reflected that we were just now reaching V1. As I called out 'rotate;' the aircraft was already lifting off. Captain stated the airspeeds differed. Cross checking showed that Captain and 'Standby' indicators agreed. V bars biased out of view. I reported to Tower we were 'unable RNAV.' Tower switched us to Departure and told us to tell them. Departure gave us vectors for the remaining of the SID. We ran checklists and completed the flight with CADC both on 1. Once 'both on 1;' we were able to engage autopilot and continue with RNAV navigation. I asked for and received an intermediate level off at FL220 to work the checklists. We received multiple CAWS 'overspeed' warnings (with none of the 3 airspeed indicators near the 'barber pole') and after running appropriate checklists; continued uneventful to our filed destination.