I have a Garmin 500 that I use on my road bike. The Auto Lap function is pretty much useless on the track.
The margin of error when dealing with the satellite is too great. When you start analyzing the data you will just be frustrated.
According to his Garmin, a guy at our track once hit 78mph.
Buy the optional speed/cadence sensor and program the head unit to ignore the satellite and defer to the speed sensor for speed data. Then it will be MUCH more accurate.
The satellite speed data is only good for averaging over long road rides.