Accuracy of speedometer?

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Accuracy of speedometer?

Postby ps_v10r8 » Fri Aug 03, 2012 9:30 am


Had a great day at the track yesterday. Thanks to TrackAddict I also get some great video and data about my driving. My only concern (more of a question really) is about how the speed is calculated. To gauge my lap in progress I pay attention primarily to two things - (a) corner entry speed and (b) speed at the end of the back straight. I generally find that the corner entry speed on the speedometer in the video is consistent with my expectations for the given corner but the speed at the end of the straight is significantly lower than I'm expecting. Overall the lap times are correct but I'm trying to optimize my speed vs breaking at the end of the back straight and would love to have it for comparison from lap to lap.

- Is the speed measured or calculated?
- If it is measured, where is it measured from?
- If it is calculated, is there some sort of adjustment that could be made to get a better value?
- If it is calculated, is there anything that can be done to measure it instead?

Thanks again,


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Re: Accuracy of speedometer?

Postby RaceRender LLC » Mon Aug 06, 2012 4:38 pm

The speed is coming from the GPS hardware, which typically measures that based on the signal's Doppler shift, although apps do not have any visibility or control over that process. It has been observed that the XGPS150's reported speeds tend to have a lag to it, and that varies from the amount of lag seen when using plain iPhone GPS hardware (which may have the benefit of accelerometer enhancement). What you could do is opt to use your OBD-2 interface to log the vehicle speed sensor... TrackAddict HD will still use the GPS-based speed, but if you export your session data into RaceRender, you could then double-click on your data file and choose which of the speed data fields is being used for the speed.

Keep in mind that even without a lag, the GPS speed can differ from the vehicle's speed sensor (or speedometer) due to variations in tire circumference, wheel slip, speed sensor design, etc. In a perfect world, the GPS speed would be the more accurate one that you'd want to use, but lag or signal issues can negate that benefit.
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