It's harder to define "average" nowadays than it used to be. Most typical models can hit between 80 and 120 mph. For speed you want to find the smallest diameter, highest pitch prop combo that pulls the plane the fastest. If you experiment with not enough diameter, there wont be enough thrust to safely fly the plane, so experiment by making gradual changes. The overall performance of the model can suffer enough to make you quit your search for the best speed prop for your plane.