The fuel gauge in your car works by measuring the resistance of the fuel level sender unit. The fuel level sender unit is located in the fuel tank and is connected to the fuel gauge by a wire. As the fuel level in the tank decreases, the resistance of the fuel level sender unit increases, which causes the fuel gauge to read lower.

Speedometer

The speedometer in your car works by measuring the rotation of the driveshaft. The driveshaft is connected to the transmission, which in turn is connected to the engine. As the engine RPMs increase, the driveshaft rotates faster, which causes the speedometer to read higher.

Why the Speedometer Jumps Around

When you hit a bump or go over train tracks, the sudden change in the road surface can cause the driveshaft to rotate erratically. This erratic rotation is then transmitted to the speedometer, which causes the needle to jump around.

How to Fix the Speedometer

If your speedometer is jumping around, there are a few things you can check. First, make sure that the driveshaft is properly connected to the transmission and that there is no play in the connection. If the driveshaft is loose, it can cause the speedometer to jump around.

Next, check the speedometer cable. The speedometer cable is responsible for transmitting the rotation of the driveshaft to the speedometer. If the speedometer cable is damaged or kinked, it can cause the speedometer to jump around.

Finally, check the speedometer itself. If the speedometer is damaged, it will need to be replaced.