homemade tachometer (rpm gauge)

Here are some instructions on making a homemade tach on instructables.com

http://www.instructables.com/id/Tachometer-made-from-a-bicycle-speedometer-cycloc/

You just need a cheap $15 - $20 bicycle speedometer and set the wheel size to be 268 mm or 27 cm. The magnet is attached to the wheel or shaft you are measuring. Its diameter doesn't matter. Then the mph reading times 100 is the rpm. For example, 28.1 mph = 2,810 rpms