Sometimes my speedometer works sometimes it doesn't. Why is this?

So, the ABS sensors tell the car how fast it is going. They are called wheel speed sensors. The data from those sensors is sent to the ECU to be relayed to the speedometer. When the information reaches the speedometer, it will adjust the needle on the dash. Either a wheel speed sensor, or, more likely the speedometer sensor is bad. Check all the fuses first, then check all the connections, then test the speedometer sensor, then the wheel sensors.