This is just the highlevel planning I'm using to design my speedometer based on a vehicle speed signal output. If you see any glaring or enormous errors or a better way to do this, let me know. My chip of choice, since its what I have, is a 16F628A. A 2007 Ford Crown Victoria has a vss wire in easy access that is not being used so that will be used.
My best information, so far, is that the vss either pulses 4000 or 8000 times per mile. Based on 1hour = 3600sec, the vss would pulse once at .9sec (3600/4000) Therefore, if my final output were simply a count of pulses until .9sec I should not need to do any math, just put the number on the lcd display. If it were to pulse 8k/sec then just divide the number by 2.
.9seconds, though, is too slow of a refresh rate. I was thinking of having 9 counters. The idea being, start each one .1sec later than the prior one. When the first gets to .9sec take it out, manipulate it, display it, clear it, go to the next one. This will assure that the counter being manipulated is the counter of the last .9sec but will be refreshed every .1sec. The vss would be on the int pin.
Again this is just thinking right now, but I figured I'd run it by you guys and see what you think or maybe suggest.