This tool is used to determine rotator dwell, engine RPM at idle, and voltage. It is limited to around 1500 RPMs's but is far more accurate than a dashboard tach. If your car does not have a tachometer to assist you in tuning your idle setting to the recommended RPM, you can install an under-hood version for tuning purposes. It should be a simple connection for engines that have ignitions that use a single coil and distributor setup. One wire to ground and one more to the negative side of the coil.