After talking to the XR folks,this pretty much sums up what they said,the XR650r will break 100 on any terrain,whearas the 450x needs complete flat ground to break 100 mph. That's straight inaccurate info. The XR will not break 100mph on "any" surface... let alone loose dirt or a graded road. My mod'd XR was built to do 110... it only did 105mph on asphalt, and with a slight downhill. It wasn't even close to a stock bike. Anyone who tells you... they maintained over 100 for a lengthy time, (and not just 'hit' a 100) needs to prove it. Video, GPS... whatever. I'd like to see the ground they did the speed, and for how long. Call me skeptical... but all in all, it's reality. But that goes for most "dirt" bikes, in dirt, and over anything but smooth terrain.