I've use this style of tachometer on an ultralight aircraft. It's accurate, small and reasonably priced at about $65.
It has a ground wire (white) and a sensor wire (red).
I've grounded to an engine mount bolt and the sensor is wrapped four times around the spark plug wire.
My mounting hardware is stainless steel: a cap screw for aesthetics, small washer to distribute the pressure to avoid cracking the plastic housing, fender washer to keep the rubber washer from mushrooming when tightened down, the rubber washer acts as a shock mount (tach doesn't contact the frame) and all secured with a nyloc nut.
The unit is mounted thru existing manufacturing holes in the frame. It's hardly noticeable when installed, but easy to read when adjusting idle speed.
This installation was a result of my inability to adjust my idle by sound, which many of you seem able to do. My hearing just isn't that great. My son-in-law said it wasn't a big deal to do so, as he has done it many times in his young life
on atvs, dirt bikes, etc. So, reluctantly, I let him do the adjusting about 3,000 miles ago. Now I know why my recent spark plug inspection revealed hotter conditions. When I fired-up the S40, new tach's initial read-out was about 6,500 rpms!
I've reset it so it's running 1,150-1,220 rpms now. I'm interested to see what the plug looks like at the next maintenance interval.