The RPM Speed Sensor is designed to mount on the hub of a bicycle wheel. It measures the rotational speed of the wheel and multiplies it by the size of the wheel to yield speed. Most spin bikes use a solid flywheel to simulate the resistance on which the RPM Speed Sensor can not be properly mounted.
If you are able to find a way to mount the RPM Speed Sensor, most spin bike flywheels do not spin at the same rate as a standard road wheel. Finding the proper wheel circumference to simulate an outdoor speed may be a bit difficult.
Unfortunately, the RPM Speed sensor is not compatible with most spin bikes.