Dashcam records at very high bitrate. Lower disk usage in exchange for much lower quality. As low as you can go while still making out enough license plates.

 nice -19 parallel --ungroup -j4 -v -q ffmpeg -i "{}" -c:v libsvtav1 -crf 43 -preset 2 -threads 8 -svtav1-params mbr=700k:aq-mode=2:enable-variance-boost=1:enable-qm=1:qm-min=0:qm-max=15:film-grain=10:keyint=30s:sharpness=3 -g 300 -map 0 -y "{.}.mkv" ::: **.mp4
 

NOTE: if on Tesla's, extract SEI metadata first before converting!