Rim width on the rear is not as important as using the proper width tire for the rim. If you put a wide tire on a narrow rim you will deform the profile and that could result in weird turning behavior. Too narrow a tire on a wide rim may not seat fully and allow it to slip. Don't worry if your front and rear tires are the same size, most bikes have oversized rears that only serve to look cool but in reality make the bike slower turning and sluggish. With a 250 you won't be worrying about spinining the rear up so a smaller tire may make the bike easier to turn with no drawbacks. That does not happen often!
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