The 17,18,19 is only the height of the rim and not the width. The larger rims usually usually holds a wider tire. You can decode the tire size by doing the following. 255/50/19 would be:

Width: 255mm wide or 10.0394 inches
Sidewall: 50% of the width is the side wall so 255*.50 = 127.5mm or 5.0197 inchs
Rim Size: 19 inches across

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