Am thinking about building a gas mileage gauge for the car. It has the
famous 3800 GM Engine in it (it's an Olds 1988 Delta 88). I get good mileage
readings using the "filler her up and see how many litres go in" method, but
I'd like a real time version.

I want to do this with the absolute least amount of "modification". So, the
way I want to do it is this. To get the gas mileage I need to know how many
litres are used per a certain distance. Distance is easy, the ECM gets a
signal from the transaxle that tells it how far it's gone.

Litres used it harder. The least intrusive way I believe is to measure the
injector pulse width. Since the pressure in the fuel rail is pretty much
constant (and easy to measure if needed), all I need is a relation between
injector pulse width and amount injected. I guess if I had the diameter of
the injector port I could get a rough idea but I'm wondering if anyone knows
of a more accurate way (I doubt the relation is that linear considering a
mass is moving). Does GM publish the pulse width vs. gas injected curves?
Anybody ever see these curves. Maybe the ECM gets a different signal where I
could determine gas used? (the mass airflow sensor, is there a relation
there I'm missing?).

Thanks for any insight. TTYL

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