Three possible reasons. Gear in transmission which is plastic could be broke. Cable itself broke or speedometer head in dash broke. Good luck.
Possible reasons for an electric bike speedometer not working could include a faulty sensor, loose wiring connections, a dead battery, or a malfunctioning display unit.
You are not moving!
The speedometer.
you're not moving?
it's asleep
Some possible reasons for a stationary bike not moving could be a loose or broken belt, a malfunctioning resistance mechanism, a problem with the pedals or crank, or issues with the electronic components such as the display or power source.
Broken speedometer cable.
Your speedometer is broken.
Speedometer cable (most common) Speedometer head Gear on transmission end of speedometer cable
Assuming this happens while you are moving you probably need to replace the speedometer cable.
speedometer
Possible reasons your speedometer not working: 1. The vehicle speed sensor has failed. 2. The speedometer itself has failed. 3. There is a failure in the wiring or connectors. 4. The engine control module has failed. I don't know what you mean by "fuel mix", please clarify.