Two traces made on same journey by car.
Some roads were primary roads with 40-70 km/h and some residential areas with no more than 30km/h.
Garmin HCx (Tracking in fastest auto-mode):
iPhone (set to track in some fast auto-mode I don't know):
If you compare the two, you will see that the iPhone always has the "outer" curve when the road curves, while the Garmin sometimes "cuts" sharp corners.
Seems like the iPhone is a bit "ahead" of itself, while the Garmin doesnt always "react" to curves that well.
Still I believe that the track from the Garmin is the most accurate as it gives more detail in lower speed residential areas.