This is an electronic device that allows just about any current 12 volt negative ground tachometer to be run on a 6 volt positive ground vehicle. It is simple to install and connect and will drive just about any tachometer from a $10 swap meet "special" to a $600 Autometer with memory and playback. These have been extensively tested and have proved accurate and reliable. They have sophisticated circuitry to "clean up" the power anomalies present in vintage vehicle electrical systems. In comes with a comprehensive instruction and installation manual. Here is a link to a short video of the device in action : . The engine is a '51 Mercury mounted in my test stand. The video starts with the engine off. The engine is started and the tachometer comes to life. The engine is revved a couple of times, showing the tachometer reacting. It then scans down to the device itself, and then scans down, showing the 6 volt Optima battery powering the whole thing. It finishes by showing the engine. If you have any questions, please contact me.