Speed is substantially incorrect at times


The speed is substantially wrong in some places in an exported video and exported logfile. It looks very correct most of the lap, then in some places slowly drops too low and then (via magically high acceleration) picks up to the correct speed again. I'm very unhappy not to be able to trust the logfile data.

Graph: https://rejsa.nu/forum/viewtopic.php?p=1524352#1524352
Corresponding video: https://rejsa.nu/forum/viewtopic.php?p=1524351#1524351

Would be very grateful if this could be looked in to.

/Magnus Thomé

My setup:
Android 6.01 on a OnePlus One phone
Qstarz BT-Q818XT
BMW M2 -16
Circuit Tools 2 to look at logs


  • aolaol
    edited April 2018
    @MagnusThome Hi Magnus,

    RaceChrono data is only as good as your GPS is able to produce. This data comes from your GPS, and RaceChrono just saves and displays that data. BT-Q818XT is quite old receiver, and there are better ones available. Maybe consider purchasing Garmin GLO which will produce much better data, as it is locked to GPS as well as GLONASS satellites.

    With careful GPS receiver placement you can sometimes improve reception, but on a powerful car like yours, and GPS behind the windshield, your BT-Q818XT receiver will always have problems like you're having now...
  • Thank you!!
  • @aol
    I'm using Garmin GLO for GPS and BT ELM327 for OBD2. I find that the GPS speed is often delayed relative to the OBD2 speed (somewhat expected) and the speed difference can be 5-10km/h. Having said that, would you said the OBD2 speed is the most accurate data?
  • @xmen15 OBD-II speed is not accurate. But it's reliable. Not accurate due to its dependency on certain wheel circumference, so it will always have certain bias. But it is reliable as it does not suffer from signal problems.

    A good GPS with good update rate and signal will probably be better.
  • @aol Not accurate even when the wheel circumference is stock?
  • @xmen, not always, but it all depends on the vehicle.
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