[14stu]

Originally Posted by TofuJoe

You'll get much more accurate data on the track with an external GPS. At high speeds, a phone's built-in GPS is not enough. Very easy to add an external:

XGPS150 is 5Hz (5 times per second) and $82: http://amzn.to/2pncia7
XGPS160 is 10Hz (10 times per second) and $135: http://amzn.to/2q7JCzB

I use the cheaper XGPS150 and it works well.

All the phone gps that I am aware of are 1Hz, which isn't quite high enough resolution for data acquisition (you can't really tell lap to lap differences). You will definitely want an external gps unit.