How about disconnect the return line, put it into a big container and drive for a minute, then measure? From what I've read, in a diesel, fuel returned is in the cicinity of 10 to 20 times fuel consumed.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked