If you exceed your speedometer's maximum speed, the pointer will remain at the maximum speed position, and will not move below the maximum speed position until your speed drops back into a readable range.
Luckily for all of the speed-fiends out there, this will NOT damage the speedometer.
If the speed exceeds the analog range and you are using a GPS speedometer with an LCD display then you can use the digital speed display feature to view your current speed.