Car tyres carry a wealth of information on the side wall, most of which you don’t need to worry about but it’s worth understanding the codes for a few reasons. The main code (e.g. 175/65 R14T) on the tyre refers to its size, composition and capabilities. The first part refers to the width in mm and the sidewall height as a percentage of the width. Next the letter refers to the construction of the tyre (here it’s radial). This is important to note as mixing different constructions of tyre on the same axel is illegal. The next number is the diameter of the interior of the tyre in inches. Finally the letter corresponds to the tyres speed rating (here, T is a maximum of 118mph)
Elsewhere, the DOT code gives information about the tyres compliance with regulations and its place of manufacture. The final four digits of this code are important to note as it refers to the week and year of manufacture (e.g. 4003 would be the 40th week of 2003). Tyres have a maximum lifespan of 10 years (less if they have never been used) after which they may be unsafe.