GPS aside, its fairly safe to assume that the rev counter is accurate at those speeds as its not fluctuating and gears are fixed, so gives you a ball park figure as well likely to be most accurate.
I've only seen my Golf round to 120-ish but feels like it would go faster, dunno about 160 quicker, it must take a long time to get that far as its not the most aero dynamic of shapes and its more about power to get up to those speeds.
My RS4 could get round to 160 pretty well but then that had over 420bhp