Buy a scangauge instead. They have a digital RPM readout and you can also keep track of other things like fuel economy, battery voltage, and water temp 
Scanguage II
It costs more than that tiny tach thing you asked about, but the quality is great and its a good brand. I have one and love it.
Scanguage II
It costs more than that tiny tach thing you asked about, but the quality is great and its a good brand. I have one and love it.
