Speed
Tap Start, mount your phone and stop the car. It arms itself, times the launch on its own and reads the result out loud. GPS and the accelerometer together, with the GPS delay measured and taken out.
Use a track or a closed road. Mount the phone in a cradle or on the dash; never hold it. Pick somewhere level: a run on more than a 1 % slope is flagged, because it isn’t comparable.
Give it a clear view of the sky for a minute first, so the GPS settles. Allow location, and on an iPhone allow motion too.
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