Same speed off flat ground as Mound.?

I haven't played baseball in years (im 20) and i can throw about 75mph on flat ground. No mound..just all arm. When i step on a mound i throw about the same speed. Shouldn't I be throwing at least 80mph off the mound if i can throw 75 with just my arm? On average how much speed does proper mechanics and using the mound effectively add?