Well, that it's a difficult question to answer, depends on a lot of the gps phone or the external antenna you are using, generally
phone that uses
glonass and gps together should be relatively accurate,
There is another factor to take into account and it's the refresh rate , i think all phones in the world at this moment works at 1 hz , that is 1 sample per second, often too slow for a racing proposes, also when you see the telemetry overlaid onto video you notice a kind of "lag" in the gauges and that
The external antenas can reach extremely precise racing lines together with a high refresh,5 hz to 10hz or 20hz even you have $$$
We have a lot of antenas and after that, we high the recommed
Garmin GLO or
XGPS160, they are the best antennas we have tested so far (with reasonable price)
There are another good and cheaper antenna, the
Q818XT but IMHO worth spending a little more and buy the other