I just read this thread and for awhile I have also been thinking of ways to make a simple mpg computer for non-obd2. Basically one would need to use the VSS or driveshaft magnet or something to get speed. And a flowmeter on a section of fuel line that is between the regulator and engine. If there is no such line, then either abandon the factory regulator and add a universal regulator a little ways back from the engine; or put a flowmeter on both the pressure line and return line and subtract the return line signal.
Ive been thinking on how to make a simple flowmeter.
OMEGA ENGINEERING - Flow Control is a good site for many types. I am thinking of an idea like the electromagnetic one. I have been reading that pure gasoline does not conduct electricity very well, but ethanol does. And isnt gas with 10% ethanol pretty much the standard these days? How about using a non-metallic section of line and wrapping it to make an electromagnet. Then forward of that a bit, wrap it with some amount of turns to pick up voltage as the charged flow goes through it. Probably a lot more turns to be more sensitive, but I dont know, electronics is not my strong point. If you have a fuel return line then wrap it in reverse to get a negative voltage, and run them in series. Then with the speed signal youll have 2 inputs for some sort of computer. Maybe a cheap small laptop or mpguino or something like that. Of course you would have to calibrate the thing but I think it might work pretty well.