Hi Todd,
No apologies needed.
A 17 inch tyre means it fits on a 17 inch diameter wheel (in caps) - not that the tyre is 17 inch (outside) diameter. The all important consideration is the outside diameter - rolling diameter as Trevor refers to it - of wheel + tyre. This what ‘controls’ the correct indicated road speed etc.
Now hopefully the logic follows that if you keep the tyre width the same yet reduce the profile (a % of the tyre width) then the outcome is you have reduced the outside dia (Trevor’s rolling diameter) of wheel + tyre, because the ‘height’ of the tyre wall has reduced.
In effect, you will find that low profile tyres are fitted to larger diameter wheels, thereby maintaining a constant outside diameter.
To do it properly you should consider 18 inch wheels if you want to move to 45 profile tyres - or as I say increase the width of the tyre - which is seldom practical.
Question remains:- Why reduce the profile?
Kind regards,
Gareth.