Looking to buy new tires for my bike. I'm pretty sure I want to get some Michelin S12's. But I don't understand how this tire sizing stuff works! My bike, a 2001 KX250 currently has a 110/90-19 on the rear, and a 80/100-21 on the front! Okay, I found info saying to replace these tires with a 130/70-19 for the rear and a 90/90-21 for the front. I don't understand. Why is this? Matt