What it does is measure RF noise so it really doesnt matter which wire you hold it close to as long as it is only reading the signal from one, the closer to the plug is probably best. Example if you get a reading of 25hz thats 25 cycles per second multipy that by 60 seconds equals 1500 rpms on a two stroke. For a four stroke you multiply by 120.
25hz equals 50 revolutions per second times 60 equals 3000rpm. Its easier to just multiply Hz by 120.
25hz equals 50 revolutions per second times 60 equals 3000rpm. Its easier to just multiply Hz by 120.