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Larger Tires — 150 on a 250R | Kawasaki Ninja 250R Parts, Reviews, Mods, and Information
Seems you can fit a 150 size tire but most recommend 140 for the rear on the new 250.
As for stock tires being crap, what comes stock on the bike? I always find this funny because chances are the ones that come stock people will say suck, but find another forum and another motorcyclist that run those after market and they think they are great tires *shrug* Most riders could not tell a difference in tires anyhow.