dual purpose tire advice please. just bought a 2007 wr 450 f with off road tires. has baja package for street legal however i need to buy tires. Plan to ride 80% off road in mid to soft terrain. manual says use 80/100-21 and 110/100-18. Can i tweak those numbers a bit? i am looking at the Pirelli MT 21 and scorpion, and the dunlop D606.