If you're in the somewhat unlikely predicament of having a car with no tachometer but you do have an oscilloscope handy, this hack is for you. I'm actually in that situation, but my 'scope is so old I'm not sure if I can make this work. Even so, it's pretty fascinating.
Oh, and this guy is using a DeLorean as his test car, so that right there is reason enough to watch this.
Essentially, you're checking voltage out of the 12V socket and looking for the inductive spikes caused by the ignition coil firing the plugs. Find out how many spikes you get per second, do a bit of math based on the number of cylinders, decide if your engine fires in regular or uneven intervals, and there you go! The hardest way to RPMs possible!
The signal is pretty noisy, but someone smarter than me could probably use an Arduino to ReadAnalogVoltage with the proper filtering hardware, and do the math on the Arduino to get a result that could be displayed in some interesting way.
Or, I guess you could buy a tach kit. But that sounds boring.