It remains constant.
That was one answer to a multiple answer question... although perhaps it was enough I like to clarify it further.
Yes... if you drop the speed to 10% of top speed you also drop the fuel usage to 10% but...
No.. fuel efficiency of the engine is constant.
This means that no matter what speed you use you will always burn the same amount of fuel for the same amount of distance traveled.
Even if space is a vacuum and have no resistance engined should perhaps have a slight curved fuel usage depending on speed used. That would probably be realistic, but I don't think this is particular important to be honest.
