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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.