Er... I believe the gearing of the Mk5 allows for a maximum of 245 km/h (152 mph) for a manual 'box or 243 km/h (151 mph) for DSG. Anyone claiming to exceed this without changing their gearbox is talking cobblers.
Your speedo may be 5-10% inaccurate at those velocities.
Your GPS will provide a considerably more accurate, but still wrong, estimation of velocity (it will be a vector remember). DaveB1970 is correct in saying that using a GPS as a reference for this will still result in an inaccurate calculation. Apart from the triangulation issue, there is a fair amount of correction for latency which will have to be applied, resulting in a margin of error that grows as the velocity vector increases. Mater may be right about how a stationary fix might be the same on a "consumer" GPS device as with a "professional" or military-grade device, but a stationary fix is not so important. There is a latency which is the transmission of GPS signal from the satellite to reception by the receiving device; this is compounded using more GPS satellites (which does allow for more accuracy/precision). There is then some calculation time which in most consumer electronics is achieved by using a look-up table or simple algorithm which will be calibrated (due to limitations of processing power available) for "normal" speeds. As the velocity vector increases the error in calculation grows larger. This is why military-based GPS systems, especially those used for guidance of ballistic or cruise missiles, can cost in the tens of thousands of dollars, and why GPS (or other GNSS)-based safety-critical applications are not available.
The best (and only real) way to determine speed (or velocity) will be using an absolute fixed reference with a calibrated chronometer (or tachymeter).
...and the headlights come on at 130km/h. Says so in the manual.
