Cheap ain't good and good ain't cheap.
If you are going to buy one, get one that's a little more expensive. Look for one that only gives the RPMs.
I bought this one.
Amazon.com: Runleader RL-HM019J Digital Tachometer with Alligator Clip, Max RPM Display, Waterproof Design. : Patio, Lawn & Garden
www.amazon.com