On 2003-11-13 18:25:46 -0800, (Jonas Heisenberg) said:

They had cruising speeds/fuel consumption of 780-850 km/h and 118-125
g/t-km. Now my question: would lowering the cruising speed to e.g.
300-400 km/h make them more efficient?

Most aircraft have a "long range" cruise speed that is less than their maximum cruise speed. But it is more on the order of around 90% of maximum cruise. At the speeds you suggested, they would likely burn even more fuel, and it might even be slower than their minimum controllable airspeed.

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Larry Fransson
Seattle, WA