On the sidewall of the tyre will be a series of numbers and letters (see diagram below):

Tyre Width: This is the width of the tyre, measured in millimetres

Tyre Profile: This is the height of tyre tyre sidewall, measured as % of the tyre width

Rim Size: This is the internal diameter of the tyre, measured in inches

Load Rating: This number references the Load Rating Table (click to view), and shows its load capacity

Speed Rating: The letter references the Speed Rating table (click to view), and shows its maximum speed allowed (if you have V rated tyres on your car you cannot fit H rated. However if you have H rated tyres on your car you can fit V rated)