Does anyone know of a real time mileage gauge/instrument that can be fitted to an '85 300 D?

I have seen gauges (ScanGauge II) for 1996 and later models.

Years ago I got from JC Whitney an on-board computer that combined a speed sensor and differential flow meter to determine instantaneous mpg. I haven't seen anything like it since.

A reasonable facsimile is a boost gauge calibrated in mpg like how BMWs and some MB gassers have a vacuum gauge calibrated in mpg. It's going to take a lot of long trips to calibrate the gauge.

Sixto
87 300D