GPS, Car speedo, or roadside speed indicator. Which do I believe?
Your GPS tracks your location with an accuracy of 3 meters. So 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. The roadside radar device (it’s not laser) does not read in fractions, so it will round up to the nearest 1 MPH. Also, the radar horns in those things aren’t state-of-the-art. The cops have better ones, and they are required to calibrate them daily. Even then, they can only approximate the speed of an approaching vehicle because of cosine error. The closer you are to the horn, the more the error. However, it’s going to give you a more accurate reading than your car speedometer. 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 g
