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My bike has 27,5" wheels and the rear tire is due for replacement. Will a 28" tire stay on a 27,5 MTB wheel once inflated to 30+ PSI? This is not a tubeless tire, it has inner tube. Has anyone tried it? I have done enough research to understand that it is not recommended, I am just curious if the tire would stay inside the rim bead seat. (Reason for even considering this: major price difference)

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The bead/rim diameter difference between the two sizes is 38mm (622mm - 584mm) or 1 1/2in. So no, there is no way to even fit a 28" tire on a 27.5" rim, as the bead diameter of a 28" tire is even larger than the outer diameter of a 27.5" rim.

The confusion may arise because 27.5", 28" and 29" are considered to be the outer diameter of the wheel - tire included. 27.5" and 29" are coming from the MTB world, that assume large tires (2" or more). 28" is a marketing denomination for road/gravel wheels (with tires narrower than 2"), but the bead diameter for 28" and 29" wheels is the same, the difference in outer diameter is only attributable to a difference in tire size.

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