Review on 🕒 Runleader Small Engine Hour Meter & Tachometer: Maintenance Reminder, Battery Replaceable | ZTR Lawn Mower Tractor Generator Marine Outboard ATV (HM011F) by Derek Evans

It was cheap but seems to work. RPMs shown match my portable digital speedometer. However, I doubt that in the instructions. It is intended to vary the number of turns around the spark plug wire to get the correct RPM. First, if you don't have any other means of measuring RPM, how the heck do you know about it? Second, an intermittent high voltage in the connector wire induces an intermittent low voltage in the sensor wire (the wire you wrap around the connector wire). The only way to determine the number of revolutions per minute is to count the heart rate. Winding more or fewer turns doesn't change the frequency at all. A spark corresponds to an impulse. The frequency is directly proportional to the speed. The strength of the impulse can be changed, but the strength does not indicate the number of revolutions per minute. So what are they talking about? Well luckily it doesn't matter, it works well and it couldn't have cost less so 5 stars.