Hello,
Thank you for your question.
To make FMB120 or FMB964 more accurate on speed detection I would suggest to use LV-CAN200 or OBD dongle to read speed data straight from CAN line. Also in configurator I would suggest then to set speed source: OBD /CAN. If speed source depends on GNSS, speed accuracy will depend on GNSS signal.
Hope this will help.
Regards.