Hi valve-n-head,
Curious that this should crop up,
many years ago i made a 'mile-age indicator' for my old Mk2 Jag.
I looked at that frequency thing,
i dont think that will be of any help to you.
https://www.wenzel.com/documents/freqdiffdivide.htm
The arrangement i used on that old Jag was really quite simple,
although i didn't set the mileage very accurately,
cos i was not really concerned too much with the mileage,
i was more concerned with improving the mileage, and it did that
very well indeed.
Dialtone makes a good point about needing the right set of input
information, although the way i see it you only need two sets of
inputted information,
namely: distance and fuel.
In the arrangement i made i had an extra wire added in to my fuel
pump, and i fitted a light sensitive transistor arrangement into
my speedo-meter housing. The light sensor was set close to the
spinning part driven by the speedo cable, this i had sprayed
matt black and i had put white marks on it with a tippex marker.
(thats a white-out paint for typing errors in a tiny bottle with
a little brush in the lid like a fingernail varnish bottle)
The pulse from the pump i used as my fuel monitor.
The pulses from the speedo i used as my distance monitor.
The indicator was, and still is, a couple of seven segment displays.
Although the unit has not been on the vehicle for many years, i
still have it knocking about somewhere ...
It was thrown together on a small 'bread-board kit' one of those
where you poke the component wires into a matrix-work of inter
connected holes.
Initially i thought that having it count up to the MPG, then doing
it again and again would be good enough for me to look at, and i
would just take notice of the final count each time.
Well that was a waste of time ... i needed it to stay at the count
number while i was driving! It was no good counting up continually.
So i modified the circuit a bit. The chips then included a buffer,
and the count from the speedo was fed through to the buffer opon
receipt of the fuel pump pulse. The trailing side of the fuel pump
pulse re-set the counter.
So that was much more use-able and see-able, the pump operated at
about two times a second at first, then when the auto choke dropped
out, went down dramatically to about four second intervals at
about its best tickover.
The pair of seven segment displays showed the count, as last put
in the buffer, and updated at each pump operation.
I couldn't be bothered to sort it out properly, and i had a common
resistor in the common line of the displays, instead of a resistor
in each leg as it should be. This meant that the 'one' was bright,
and the 'eight' was dim, but i did not care, i used it like that
for years.
I will try to set out the principle as clearly as i can, in case
it is still not clear. The unit does no calculations. The unit
counts the white marks on the speedo internal spinning disc which
is its reference to the car wheels. The count is forwarded to the
display each time the fuel pump operates. The fuel pump trailing
side pulse zeroes the counter.
So, on tick-over, standing at traffic lights, the fuel pump would
operate many times, and each time would forward zero to the display.
In overdrive, at a decent speed, i think about forty to fifty miles
per hour gave me the highest readings, thats the number of pulses
from the speedo, forwarded to the display at each operation of the
fuel pump.
I never got around to adjusting the number of white marks inside
the speedo, which could have been made to correspond to MPG,
cos i was more interested in getting it as high as i could.
Based on the mile-ometer and the gallons that went in, i reckoned
at about thirty five to forty miles to the gallon. Not bad for an
old 3.8 Mk2 Jag.
I will see if i can find the old board, and post a few photos.
It was pretty basic and simple.
If i find it, then it will have the chips, so i could post their
numbers too if you want. There weren't many components involved
as i recall, but i often don't remember things quite right, so
i will have a look when i find it.
Regards, John