the 7J (or 8J etc...) stamped on your wheel is the width of the wheel in inches. Multiply this value by 25.4 to find out the width in millimetres (which directly translates to the tyre width).
eg
7x25.4=177.8
7.5x25.4=190.5
8x25.4=203.2
This is the width of the alloy wheel. as fitted to your car. Lets take my car as an example. It is fitted with 7.5J wheels which transates to near 190mm width across the tyre bead. i cant say to know why, but going +20 to 30mm oversize on the tyre compared to wheel width seems to give a very good, neutral stance. I guess this is down to the shoulder of the tyre absorbing any discrepancy between the wheel and tyre width.
point to be learned, you cant fit wide tyres on narrow rims - and vice versa.
IMO, of course
</sucking eggs>