A basic understanding of tyre dimensions is important when buying new tyres, and even more so when considering changing to a different size. The size description contains a number of indicators that will allow you to easily find tyres that fit your vehicle. 225/45 R18 tyres are some of the most popular sizes for mid-range and premium cars.
Opening this description, the number 225 represents the tyre's cross-section width, expressed in millimetres. This is essentially the distance from sidewall to sidewall, at the point where they are most distant from each other. This indication is connected to the next one - the umber 45, which is the aspect ratio.
Simply put, an aspect ratio of 45 means that the sidewall's height (the distance from tread line to rim line) is equal to 45% of cross-section width (here, of 195 mm). Next, we are informed that this is a radial tyre, thanks to the R' marking. Closing this description, the number 18 tells us that this tyre will fit 18 inch rims.