It is a digital speedgauge app which uses GPS system.
So you will need an active GPS connection for the app work.
You can use this app with bike smartphone holder and anything else, especially HUD with your car.
HUD means Head Up Display.

It shows you total distance, trip meter, max and current speed.
Kilometers(km/h), miles(mph) and Nautical mile(kts,knots,kn) are supporeted in this app.

NOTE: Due to phone GPS limitation, It may not work well in cloudy weather or indoor places.
And I don't guarantee that it shows accurate data because of GPS errors.