Well, My 1984 Civic Wagon used the same signal for the coil and tach. I'm guessing most cars with a distributor are the same. On a four stroke engine, all cylinders will fire every two revolutions of the crank. So that's two pulses per revolution with a four cylinder engine. Multiply by the engine speed to get pulses per minute, then divide by 60 to get pulses per second, or Hertz.

So:
2 pulses x 8000 rpm / 60 = 267 Hz

I don't know if it's correct, but it makes sense to me. The signal is negative, by the way (or from the negative coil terminal, anyway).