Sorry Moderators - I need a quick answer, so I had to put it in General!

Can anybody tell me what the first two numbers refer to EXACTLY, when dealing with tyre sizes?

eg. 215/50 16
eg. 225/40 17

etc.

Thanks.

 

Found this...

for P205/75 R15 S

P - a passenger vehicle tyre.
205 - the width of the tyre (section) in millimetres.
75 - The aspect ratio of the width of the tyre against the sidewall height.
R - Radial construction tyre
15 - The wheel diameter in inches.
S - The manufacturer has recommended that the tyre should not be used on applications faster than 112mph.

 

In your example...215/50 16

215 - is the tyre width in mm (i.e. 21.5cm)
50 - is the percentage height of the tyre (from the rim to the tread, in this case 50%, i.e. 10.75cm)
16 - is the wheel rim size, i.e 16 inch wheel.

hth.

 

First is the width of the tyre in millimetres.
Second is the 'cross section', i.e. the percentage of the width that the height of the tyre is.

For your example:
215/50 is 215mm across the tread, 107.5mm from inner to outer edge of the tyre (50% of 215)
225/40 is 225mm across the tread, 90mm from inner to outer edge of the tyre (40% of 225)

16 and 17 are just what size wheels they fit on.

So the overall wheel diameter is (wheel diameter in inches * 25.4)+2*(tyre width*profile). You need to keep the overall diameter the same/similar as you change wheel and tyre sizes.