A knobbly tyre's appearance might lead you to believe it has a larger diameter and width than an equivalent size road tyre, but all tyre measurements are quoted to the extremities (ie: the edge of the tread/tip of a knobble). A 90/90-21 tyre should (in theory) have exactly the same clearance whatever the tread pattern/design. The width is 90mm (at its widest part) and the height is 90% of the width (at its tallest part), whatever the tyre/tread design.

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