I use a DL1 datalogger from
www.race-technology.co.ukrecord the in-car video on a Panasonic DV videocam then download it to a PC using firewire, then join it all together using
www.trackvision.netThe only modification I made to the car was to run a tach signal to the datalogger which then works out everything else (HP, torque, gear, ...).
While it produces some really cool video, I use it to learn to drive better. Before a track day I spend about half the day watching the video of the best laps I have done at that circuit and try to see where I could do better (lines, braking, shift points, gears selection, different compromises, ...).