If the tachometer uses a pwm signal, an RC circuit will filter it to an analog voltage. If you attach a small capacitor across the signal and ground wires, this should produce an analog voltage. Depending on the current draw, you may or may not need an amplifer circuit for this. If you do need an amplifier, look for an circuit that will amplify the current with an op-amp. Try starting with
this site for the basics. You should be able to do this easily. You can also do interesting things with the op-amp such as add and subtract voltages. The amplifier is simple to build using only a couple dollars of parts (Maybe a little more expensive if you get them at radio-shack). I hope this helps you out.
I think circuit in the car tachometer link using the 555 chip turn's frequency into an analog voltage. A PWM signal has the same frequency regardless of the input. What changes is how much time per cycle is at the high voltage and how much is at the low.