Should you need to find out the size of your tyre just take a look at the side of your tyre. On every tyre you will find a sequence of numbers and letters.
What they stand for :-
205 – Is the Width
55 – Is the ‘profile’ (aspect ratio of the tyre)
R16 – Rimsize
91 – Is the ‘load rating’
V – Stands for ‘Speed Rating’ (the maximum speed for the tyre when fully loaded)