Magnets are very important when driving your car. First of all, they're involved with your speedometer, tachometer, galvonometer (fuel gauge), and all the other gauges on your dash boards.
This often refers to a dashboard instrument panel that houses 4 large gauges. Usually two of the gauges will be Speedometer and Tachometer. The other two may some combination of Engine oil pressure, Engine Temperature, Intake vacuum, Alternator/Battery voltage, Fuel pressure, Clock, etc.
If striking the dashboard encourages or clears the fault then either a bad earth or bad solder joints on the membrane sheet behind the dials.
Check the plastic "Molex" connector at the firewall. Disconnect and reconnect. Same thing happened on my 1894 Maxima; could drive the car but the tachometer and other gauges stopped working.
Both of these run off systems using electrical pulse from the engine/transmission. It is common for the ground from the gauges to separate from other grounds and grounded together in the dash and to the engine block. Check this grounding system.
Your speedometer may have failed, or the speedometer cable that connects to the transmission, or the plastic gear in the transmission may be broken.
If it is an electric speedo it probably is the speed sensor and if it is a manal speedo it is the cable.
I have a 1989 Pony. If you want, I can take a picture of it. In the middle there's a speedometer, white numbers are the speed in MPH, the inner circle, the blue numbers are the speed in KMH. There's only one odometer, without trip. There're any other gauges, just warning lights. There's no tachometer.
check the speed sensor.not sure where it is on that particular vehicle
check the vss located on the tailshaft of the transmission for the speedometer issue the odometer is figured out based on the speedometer also so it will quit as well. this sounds like either the sensor is bad or a short in the wire from it The cord for the speedometer came undone from the transmission or the gauge cluster is bad