The 205 in the 205/40 R17 description represents cross section width, which is measured when the tyre is mounted onto the rim, and is expressed in millimetres. While taking this measurement, the tyre should not bear any weight. Since the rim can slightly deform the tyre, and also because during the fabrication process minor variations may occur, this value is only approximate. Therefore, these tyres may in fact have a width of 206 mm, for example. This, however, is not problematic, as the next standard size is 215/40 R17.
Likewise, actual height may also present some variation. At any rate, this value is not even directly indicated in the description. The second number actually represents the tyre's aspect ratio, resulting from the ratio between the cross section's width and height. For the 205/40 R17, flank height is determined through a simple calculation: 205 mm (width) is multiplied by 0.4 (or 40%, the aspect ratio), which equals to 82 mm. Such a value places these tyres in the small aspect ratio category. This value is decisive for purchasing tyres that will properly fit their corresponding rims.