Motorcycle RPM counter
As a general point if you are connecting to the same signal that goes to the ignition coil primary you will want to change your idea. The flyback pulses from the coil are high enough voltage that they can KILL most electronic circuits that you connect there - unless you really know what you are doing and how to properly clamp and filter the signal. Your questions regarding the transistor would indicate that you may not be prepared to deal with that. 

Most vehicles of the present day have some type of RPM sensor on the engine that feeds into the ECU (Engine Control Unit). Tap into that instead.