About wider tires
Lots of new people around here ask about putting those cool, phat tires on their 250 that they see on bigger bikes and on TV. This is a really bad idea, and if you can't live with what we just said you'll be better off getting the 600 supersport that you really want.
The EX250 rear rim ('86-07) is only 2.5" wide. Anything wider than 130mm gets pinched in the rim and really compromises the handling that people think they're going to get with a wider tire.
While the EX250J has slightly wider wheels, the same guidelines apply to mounting tires that are appreciably wider than the stock sizes. Tires for the 250J
For more information on this subject: