Here's a block diagram of a pretty simple concept for an mpg meter. The concept is based on the soundcard mpg monitor program- divide the fuel injector pulsewidth into the mileage and use a scaling factor to get mpg.
I've built and tested the prototype circuits for the duty cycle to voltage converter, the frequency to voltage converter, and the signal processors, which are simple comparators. So far, so good; I get voltages proportional to duty cycle and frequency of signals generated on my workbench.
I have a schematic for a simple PWM/analog division circuit that I plan to prototype in the medium future. The whole circuit should be easy to implement with a quad opamp chip, a 555 timer, and a voltage regulator chip plus a handful of discrete components.
If anyone's interested, I'll post schematics of the circuits as I build and test them. It would be nice to be able to read mpg on a simple tachometer style meter without having to lug my notebook computer around...
