"In wind tunnel testing, by what factor is the wind speed reduced by?"
There may be times in tunnel testing a scale model that one might change the airspeed to compensate for Reynolds numbers. However in that case you would INCREASE the airspeed for the small model, in proportion to its scale. For instance, when I was involved in automotive tests of 1/4 scale models, we would run the tunnel at about 4 times the speed (i.e. 50mph full scale > 200mph for the one quarter scale model).

Allan