That being said, not all dashcams arbitrarily display the speed. Some (like mine) simply record to a normal video file with just a date and time stamp, but you can still get the speed info using their separate software.
Link to what I have: https://www.amazon.com/dp/B008EPWWF4/ref=cm_sw_r_awd_g-Movb0TK7T37
If the camera is set to record the dash display they'd have a hard time disputing that. Or one like this one, https://www.amazon.com/dp/B008EPWWF4/ref=cm_sw_r_awd_g-Movb0TK7T37, which uses GPS for speed.