GT Power RC Multi-Blade Micro 0-99999rpm Tachometer. 

Ever wanted to know how fast your motor is spinning?
Wished you could better select the right propeller for your motor?
Now you can more accurately tune and performance test your engine, motor, edf or heli with the Turnigy tachometer.

Measures 2 to 9 propellers.
Super easy to use.
0-99999rpm range

Features: 
Ultra sharp backlit LCD
220mah Lion battery included
Auto shut-off when idle. (no more flat batteries)
Weighs only 39g.

Note: For optimal accuracy please use in a well lit area (do not use fluorescent lighting)!