Mondeo grip

The internal diameter of the tyre (ie, the wheel size !) has next to no bearing on the external diameter (ie, the tread pattern part) !

His original statement is correct : "it depends on the outer circumference of the tyre, not the wheel diameter"

Your statement is incorrect : "You didn't pay attention to maths at school did you? Circumference is pretty bloody dependent on diameter"

You can make the wheel diameter LOTS bigger before you have to increase the tyre's circumference - that's why people fit lower profile tyres to their bigger alloys :)

Hope that helps !