As a rule of thumb, a diesel engine uses 0,2 litres of diesel

pr. HP/h. That is if the engine if 100 HP then you will use 80% of the total capasity in top revolutions. Assuming that this is 2000 r/min. Then you will use 16 litres/h. If you reduce the speed to 1000 r/min, the consumption will be 4 litres/h.

(Double the speed, square the consumtion, half the speed, squareroot the consumption, interpolate for values in between)