I just had a look at the mkgmap style sheets:
highway=residential [0x06 road_class=0 road_speed=2 resolution 22]
highway=minor [0x06 road_class=1 road_speed=3 resolution 21]
highway=unclassified [0x06 road_class=0 road_speed=3 resolution 21]
As you see the mkgmap standard style sheet does the job you require. If your style sheets are different, simply change the number into 0x06 if you use the standard mkgmap style sheets.