An extension ruler intended for symmetry measurements of vehicles, having a built in measuring device for the extension of the ruler and transfer of data or figures to a display. The measuring device includes a tape extending from one end of the ruler to the other end where the tape is wound on a drum spring biased in the winding direction, which drum is provided with teeth and that the passage of these is detected and counted by a light source and a photo diode and that the drum rotation detected in this way is used to provide a measure of the extending of the ruler.