[mkgmap-dev] Speed Limit Icon

Hi, i use mkgmap for some weeks now and I really like it! I have one Question: My Garmin device shows a speed limit icon in the screen with the standard map and some downloaded OSM-based-mkgmap-generated maps. But the maps I generate show never this speed limit icon??? I searched a lot but did not find out what option I have to activate? Can you halp me? Thanks, Reinhard