Here's a simple circuit that can be built with just a few parts from RadioShack. U1 is a 555 timer and U2 is an LM358 dual op-amp. Simply adjust RV1 to set the RPM threshold:



You could easily replace R5 and D1 with a 1k ohm resistor and a 2N2222 NPN transistor to switch larger loads -- such as the CEL or an LED array.