[HighlandPete]

Tyre and rim combinations are controlled by the European Tyre and Rim Technical Organisation (ETRTO).

There is a design rim or 'measuring rim' for any given tyre, but a range of tyres which can be used on a given rim width. Car designers uses chosen tyre widths (sections) on given rims to get the characteristics the makers want.

But means there is more than one possibility for tyre fitment. For example for your 225/40 R18; the design rim is 8.0", the fitting range 7.5" - 9.0".

What this means in reality, you can use 225/40 winter tyres both front and rear, on a staggered rim set (8/8.5"), as many have done.

HighlandPete