The G-tech works via one of those magical semi conductor G force sensors. As used in Airbag systems to identify sharp deceleration of say a brick wall..
Basically if it knows your stationary to start with, then it can measure your acceleration along the length of the entire 400 m and cacluate the distance and speed and it measures the time. It starts the timer as soon as it detects the first bit of acceleration which is accually a pretty neat way of starting a clock.
So speed and distance are not directly measured only calculated.
But they seem to do a pretty good job. Obviously some variation, but there are variations at diffrent 1/4 tracks as well.. Depends on the peice of road your using, friction levels, is it flat etc..
Could get some impressive times if you start at the top of a hill or from a bit of a roll.
Its basically as accurate as a 1/4 mile if used correctly. Of course people could lie, fluff take offs, be bad drivers, not do it on flat road either up hill or down hill etc. just a few degrees will make a noticible effect. Just like at eastern creek..
But convinence it sounds pretty good. Just need something flat, 400m long, legal and safe to your 1/4 speed, start from a standing start etc..
They cost a few hundread dollars.. Autospeed sells them..