Actually, the figure makes sense to me. 8 knots gives the best mileage (kilometerage
), about 34000km. I'm not completely sure but I think 7 knots would give 30000km with a full fueltank. As any speed slower than the 'best mileage' speed seems to give the same duration for the journey. Range divided by speed seems to be a constant time below the best speed.
3 weeks (21 days) continous at 7 knots get's you about 3530 NM, or 6534 km. Add to that 24000km left in the tank and voila!