Preferably, said step includes starting said {HEAD ~ speed changing operation} of said {TAIL ~ either one transmission mechanism} when a down shift command for instructing a shifting to an at least two level lower shift position is output as the speed change command 
Preferably , said step includes starting said { HEAD ~ speed changing operation } of said { TAIL ~ either one transmission mechanism } when a down shift command for instructing a shifting to an at least two level lower shift position is output as the speed change command
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH