Take a 1m x 150mm strip of bacofoil and cut long 140mm lines into it every centimetre or so (so it looks like piano keys) and attach it to your back window - crack the back window open a little to allow the draft to cause the foil to flutter.
It causes mayhem with doppler road cameras as you go past and they then record your speed. The speed print out fluctuates between 0 - 999mph.
