Pacebox is a free-to-use app developed in the home of the famous Autobahn. You can use this app simply as a speedometer, but it can do much more than that:

Acceleration measurements: track your speed and precisely measure your acceleration from 0 to 60 mph or any range and unit you want. Save your measurements and show them later to your friends - with a detail view of your measurements. Check your height difference, real time and height corrected time, GPS precision and much more! You won't need a GPS Black Box anymore with this app.

Windshield Mode: turn your phone into an interactive heads up display simply by laying it under your windshield.

Gesture Controls: no need to fiddle with your phone on high speeds. Simply swipe up to adjust your settings or swipe left for windshield mode.

Disclaimer:
Never let this App distract you from traffic. Do not use this App when driving a vehicle. Do not endanger any person or property when using this App. Always adhere to traffic regulations and the given speed limit. Do not provoke or start any races outside a closed race track. The developer and publisher of this App is neither responsible for any damages that were caused nor illegal actions that were done whilst using this App.

Pacebox is open source. You can view the code on Github:
https://github.com/PhilippMatthes/dragtimer