If your saphe drive shows inaccurate speed, then a calibration of the phone's magnetic sensor may remedy the problem. You calibrate the magnetic sensor on Android phones as follows: 

Install the GPS essentials app - Select compass - If a red vertical 8-digit appears below the compass needle then the magnetic sensor should be calibrated.Hold the phone in your hand and move it in eights appx. four times. If done correctly the red vertical 8 digit will turn gray. If it is still red, try again. 

Here is a short video showing the technique: Magnetic sensor calibration