Harry's GPS Suite Forum

Hello,

I do struggle with understanding one speed data column, as it is found in the CSV export.
I was running a recoding to compare "car instrument speed" with "real speed" (GPS reading).
For this, I've locked the car into a number of fixed speeds using the limiter, and wanted to measured OBD2 values and GPS values.

Looking at the CSV file, I've assumed column "SPEED_KPH" to correlate to GPS speed, whilst "WHEEL_SPEED_KPH" would correlate to car instrument speed.

Indeed the green column WHEEL_SPEED_KPH" matches my expectation, as I locked the car into different speeds, running "flat" when measuring.
However, the blue column SPEED_KPH consistently shows higher values above the "WHEEL_SPEED_KPH", which is the opposite of what would be expected. Usually the real speed would be lower the car's instrument speed.

In fact the GPX export from the very same recording, exported together with the .csv, shows expected values, e.g. 32.78 m/s = 118km/h where I locked the car into 120km/h. This is consistent with what I observed when comparing values using navigation apps that show GPS speed.
<trkpt lat="47.563188" lon="8.501499">
<ele>404.6</ele>
<time>2023-06-15T15:28:00.000Z</time>
<course>295.7</course>
<speed>32.78</speed>
<fix>3d</fix>
<hdop>0.4</hdop>
</trkpt>

So now my trouble is understanding, what kind of speed blue column "SPEED_KPH" actually represents.
Can somebody shed some light on this for me, please?

The two columns are GPS speed measured and wheel speed measured. The later relates to dashboard speed but may differ because speed displayed always needs to be equal or higher than real speed. So dashboard speed is probably the least accurate

When driving at a constant speed, GPS speed will be the one with highest precision. When varying speeds, GPS lags and misses short min and max values. So the best approach for proper data is calibrating OBD wheel speed to GPS and use OBD wheel speed afterwards. You need to repeat calibration from time to time. Worn tires get smaller, higher pressure makes them bigger etc. All of this will result on different OBD wheel speed memories.

Ok, taking a deeper look at the data, SPEED_KPH column actually seems to be correct,
but WHEEL_SPEED_KPH apparently is reporting lower than set in the instrument cluster.
Speed was locked in to 120 km/h here.
Probably due to apparently wrong data entered in gear ratio?
Will try to run calibration.

I guess it is a wrong assumption, "WHEEL SPEED" would report what the dashboard is saying.

When you lock your speed to 120, the speed will be controlled to match the dashboard speed. The dashboard speed however, is not the real speed. And as written above, it isn‘t the wheel speed measured either.

- Harry