205/45 R17 tyres are a popular size for larger-rimmed compact and mid-sized cars. This description gives important information regarding the tyre’s width, height, constructive type, and inner diameter. Cross-section width is indicated by the first number – in this case, the tyre is 205 millimetres wide from sidewall to sidewall.
The second number, 45, indicates cross-section height – the distance from the tread line, where the tyre contacts the ground, to the rim line, where the tyre connects with the rim. This value is called aspect ratio; an aspect ratio of 45 means that cross-section height is equal to 45% of cross-section width.
This is a radial tyre, as indicated by the letter R. Radial tyres are the norm for cars and other light vehicles, with only some vintage tyres being of the old-school bias constructive type. Also pay attention to the last number in the description, 17. This tells us that these tyres may only be used with 17 inch rims.