Method for determining wheel alignment of a vehicle, which vehicle comprises a first wheel axle having an axle end with at least one wheel member (3a-b) at a respective longitudinal side of the vehicle, the method comprising the steps of; - arranging, for each longitudinal side of the vehicle, a first (6a-b, 16a-b) and a second (6c-d, 16c-d) optical reference element, each having at least two measuring marks (21a, 21b) defining a horizontal reference line, at fixed positions at a measuring site and such that the reference lines are generally in parallel with each other; - positioning the vehicle at the measuring site, such that there, for each longitudinal side of the vehicle, is free sight between the wheel members and the respective first and second optical reference elements, - mounting an optical measuring instrument (5a-b, 15a-b) to a wheel member (3a, 3b)at each axle end of the first wheel axle, and - mounting an optical measuring instrument to a wheel member (2a, 2b) at each axle end of a second wheel axle of the vehicle or to each end of a measuring rod mounted to the vehicle, the method being characterized by the steps of; - determining a first relative position of each wheel member (2a, 2b, 3a, 3b) and each rod end to which an optical measuring instrument has been mounted, by means of the optical measuring instrument and the first and second reference elements arranged at each side of the vehicle; - determining a first toe angle of the wheel member (3a, 3b) at each axle end of the first wheel axle, by means of the optical measuring instrument and the first and second reference elements arranged at each side of the vehicle, - determining a first geometrical centreline of the vehicle based on the first relative positions, - rotating each wheel member by approximately 180°, - determining a second relative position of each wheel member (2a, 2b, 3a, 3b) and each rod end to which an optical measuring instrument has been mounted, by means of the optical measuring instrument and the first and second reference elements arranged at each side of the vehicle; - determining a second toe angle of the wheel member (3a, 3b) at each axle end of the first wheel axle at said second relative position, by means of the optical measuring instrument and the first and second reference elements arranged at each side of the vehicle - determining a second geometrical centreline of the vehicle based on the second relative positions, - calculating the out of square of the first wheel axle based on the second geometrical centreline of the vehicle, the second toe angles, the difference between the first and second geometrical centreline and the differences between the respective first and second toe angles. - arranging, for each longitudinal side of the vehicle, a first (6a-b, 16a-b) and a second (6c-d, 16c-d) optical reference element, each having at least two measuring marks (21a, 21b) defining a horizontal reference line, at fixed positions at a measuring site and such that the reference lines are generally in parallel with each other; - positioning the vehicle at the measuring site, such that there, for each longitudinal side of the vehicle, is free sight between the wheel members and the respective first and second optical reference elements, - mounting an optical measuring instrument (5a-b, 15a-b) to a wheel member (3a, 3b)at each axle end of the first wheel axle, and - mounting an optical measuring instrument to a wheel member (2a, 2b) at each axle end of a second wheel axle of the vehicle or to each end of a measuring rod mounted to the vehicle, the method being characterized by the steps of; - determining a first relative position of each wheel member (2a, 2b, 3a, 3b) and each rod end to which an optical measuring instrument has been mounted, by means of the optical measuring instrument and the first and second reference elements arranged at each side of the vehicle; - determining a first toe angle of the wheel member (3a, 3b) at each axle end of the first wheel axle, by means of the optical measuring instrument and the first and second reference elements arranged at each side of the vehicle, - determining a first geometrical centreline of the vehicle based on the first relative positions, - rotating each wheel member by approximately 180°, - determining a second relative position of each wheel member (2a, 2b, 3a, 3b) and each rod end to which an optical measuring instrument has been mounted, by means of the optical measuring instrument and the first and second reference elements arranged at each side of the vehicle; - determining a second toe angle of the wheel member (3a, 3b) at each axle end of the first wheel axle at said second relative position, by means of the optical measuring instrument and the first and second reference elements arranged at each side of the vehicle - determining a second geometrical centreline of the vehicle based on the second relative positions, - calculating the out of square of the first wheel axle based on the second geometrical centreline of the vehicle, the second toe angles, the difference between the first and second geometrical centreline and the differences between the respective first and second toe angles.