[Gav]

I have a GPS lap timer that is accurate to .01 secs apparently.
It is called 'racechrono' mine has an external GPS antenna that can go just about anywhere on the car. They are made for bikes so are light and rain proof.
It is really easy to use, all the Australian tracks are plotted and have all the sectors pre set.
Lap data, speeds can all be viewed without downloading to a PC.
It was within a hundreth or so with the timing gear at the nationals.
Ash Bright has one too if you want another opinion.