There are a number of factors so it is difficult to provide specific guidance but there are some general principles that might help.
One is that increasing speed above a certain point has a big adverse effect. A wikipedia
article explains and has more useful information.
Another is that you want to avoid slowing down as much as possible, since it there that fuel is 'wasted'.
For such a short trip I'd have thought that the time for the engine to warm up might be a significant factor as well.
there is only the road, nothing but the road ...