EVERYONE WHO OWNS OUTDOOR POWER EQUIPMENT SHOULD OWN A TACH.
I have seen Z mowers & midsize mowers turning as low as 2800 rpms and the owners just thought they were getting old and not performing as well as they did when they were new. Every engine serviced in my shop is checked with a tach before it is signed off by the service manager.
My low priced tachs of choice are built by SENDEC. They use an inductive pickup wire which can be wound into a tight coil, slightly smaller than the tach housings, and zip tied to the underside of the units. They will last for several years. Just hold the coil close to a spark plug wire to get an instant reading.