as far as I know there are roughly 3 ways of denoting tyre size on bikes -
First would be the width across the knobs,
second would be the width of the carcass (e.g the main "bag" of the tyre, not the tread)
third would be the "diameter" of the carcass, as they are roughly round.
Motorbike and car tyre widths differ a lot due to intended rim widths, and manufacturers measuring the tread width from different points - for example the GT-radial 195mm tyres I have now are narrower than my old bridgestones, mainly because they seem to have measured the tread as it "wraps" onto the sidewall, rather than across the flat of the contact patch.