This speedometer uses the GPS from your android device to measure your speed. You can adjust the maximum speed displayed according to your activity like driving a car or motorbike, cycling running walking etc.
The indicator on the center bottom of the dial becomes green when your device receives GPS information A red indicator is displayed when your current speed can not be measured.
On some devices the GPS reports the speed in regular intervals, the Nexus 4 and 5 seem to use 0.9km/h as minimum interval which result in rather "jumpy" movements of the needle when you display the speed in the interval 0-10km/h (Walking).
This app prevents your display from going into sleep mode.
The accuracy of the measured and displayed speed depends on a lot of factors like GPS (availability/quality of signal and accuracy) and the processing speed of your Android device. GPS data refreshes at a typical rate of once every second.
This app is by no means a replacement for calibrated speed measuring devices and should be used for recreational purposes only. The developer cannot be held responsible for the correctness of the information displayed.

The developer disclaims and makes no warranties, express or implied, concerning this app, including, without limitation, any implied warranties of fitness for a particular purpose.
In no event is the developer liable to you or any other person or entity for any indirect, incidental, consequential or punitive damages, including loss of profit or goodwill, for any matter arising out of the use of this app.