You could work backward from the numbers given on the data plate, assuming the engine produces rated HP at max fuel consumption and ~10 HP at idle....

Or you could perform an experiment, similar in setup to how some purge their engines. Hook up a soda bottle with a known volume of fuel to the fuel lines, pipe the return line to said bottle, run the engine for a period of time and measure the volume.

On my own SD, a 2-liter bottle full of Marvel Mystery Oil will last a long time at idle (maybe 20 mins), but I don't just let it sit - 2 liters is more than enough to drive the 3 miles down to the stoplight and back.

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1983 300SD 467,000 km - 473 champagnermetallic

location: 34°5' N 106°53' W (Socorro NM, USA)