The vehicle speed in analysis can be calculated in a number of different ways, which can be selected from the option list.
In most applications it is normally desireable to calculate speed from a combination of GPS and accelerometer data.
GPS only can be useful in applications where the acceleretomer data was not useable for some reason, the most common being that the data was recorded on a boat, or the logger was not mounted correctly – for example maybe it was just resting on a seat and allowed to move about. In these cases using the accelerometer data will cause considerable inaccuracies.
The other options use wheel speed to aid the speed calculations. For most applications this is less accurate than GPS due to problems of the wheels locking up and spinning under load, however in cases where the GPS data is poor using wheel speed is a useful option.
Note that distance is calculated by analysis based on speed, so the options that you choose for speed with also directly affect distance. Speed and distance are also used on the position calculations, so again any problems in speed will impact the track mapping.
VERY IMPORTANT: If you use wheel speed to calculate speed in the analysis software, but wheel speed is incorrectly configured, the results that you get in the analysis software will be nonsense. There is information on setting up wheel speed in the software here.