This study demonstrated that the constant average speed of a dynamic car could be measured and calculated using the smartphone magnetometer. The apparatus setup was built using a dynamic car, a linear track up to 1.50 m, a bunch of magnets, and a smartphone magnetometer application. The smartphone magnetometer application, 'Physics Toolbox Suite', was free for the experiment. The magnet and smartphone magnetometer were attached on a linear track and dynamic car, respectively. When the dynamic car are moving on the car track, the smartphone magnetometer will measure the magnetic field value versus the time relation. The magnetic field value will fluctuate, increasing when close to the magnet or decrease when the distance from the magnet increases. The magnetic field properties (peaks time) versus the magnet distance position were analyzed using linear fitting, and we find the average speed of the dynamic car. We hope that this magnetometer experiment will be valuably used in general physics laboratories.