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I ran the Bridgestone BT601 tires in for most of my mini-motard racing in the standard YSR sizes. The big tire does fit the back of the X4. That was a great tire for the summer months as they gripped like mad with just a bit of heat in them. They even had decent wet grip during the summer. The 601's were terrible at Cycles 125 in the cold and felt like riding on ice. I'd run some cheap street compound tires in the cold like a Pirelli SL26 or SL36. Most people run Michelin race tires at the real tracks. They were always sold out anytime I looked for them.