CORRECTION: missed a divide-by-dt for the +=

corrected code should be

  sensitivity = userpref.sensitivity * max_ang / ( screenwidth/2 )
  decay_rate = 6.0

  frame(dt){
    dx = mouse.x - center_pos
    mouse.reset()
    steer_ang += sensitivity*dx/dt
    steer_ang.clamp(-max_ang,max_ang)
    steer_ang *= exp(-decay_rate * dt)
  }