I've had HUD apps showing speed, but...From the BBC's Technology News section
Sat-nav firm Garmin has made a portable head-up display (HUD) for cars that projects turn-by-turn directions on to a vehicle's windscreen.
The portable HUD works with a smartphone and Garmin app to generate navigation information.
The device can project information onto a transparent film attached to the windshield. The gadget can project directional arrows, distances, current speed and speed limits on the windscreen.
Read it here: http://www.bbc.co.uk/news/technology-23226569