Your GPS tracks your location with an accuracy of couple of meters. So as long as its internal clock is accurate, it will give you the most accurate reading, even at relatively low speeds. I have a handheld GPS, and it even measures my speed when I'm walking.
Car speedometers use a sensor on the driveshaft to measure your speed as a function of wheel revolution. Because the tires are made out of rubber, their diameter varies with speed, load, and temperature, even when inflated to their recommended pressure. You'll never get an accurate reading from a car speedometer.