This paper shows, that in the vehicle the slip angle is produced by two different mechanism: the steer and the sideslip, each of which has a different function. The steer produces the lateral force acting in the direction of steer and thus controls the change of direction of vehicle's motion. The sideslip produces the lateral force opposing the change in direction of vehicle's motion. It expresses vehicle's response to control and/or disturbance inputs. The slip angle and the inclination angle are improperly defined in the SAE J670 tire axis system. Consequently the tire data related to these angles must be corrected before they can be used in vehicle dynamics computer programs. To properly define the slip angle, a new “axis system, the trajectory axis system, has been introduced, and the transformation of data from the existing axis systems to the new system is shown.