MPG readings on older vehicles?

[stillsearching]

Is there a way to get a hyper accurate fuel measurement on vehicles before all the modern OBD2 and later type stuff? Carburetors, older fuel injection, preelectronic diesels, etc.

Note i'm referring to realtime measurement.

[D.O.G.]

My Datsun falls into the "older fuel injection" category.
I have an MPGuino that I'll be fitting once I can add a reed switch to the speedo, to act as a vehicle speed sensor.
The injector signal is already present in this case of course.

For anyone interested, this link shows one way of adding a VSS.
Adding a VSS output to a Mechanical Speedo (reed switch)

Pete.