So overall diameter difference is supposed to be 11.4mm but the reality is much bigger .
Both tyres are just at the beginning of the wear bars .
That piece of wood is 22mm tall and the 17" is taller again !
It's not a one off it's noticing things over the years , a car on coilovers gaining chassis rail and front splitter clearance , lowered on 18 , having to purchase even lower springs for 19 , a R34 with correct staggered 18 looks like a 4x4 , lots of average cars upsizing and chassis rising , a guy renewing same size tyres up front and the new rub the inboard suspension , numerous pics of same size tyres with width variations and a rare find of a tyre manufacturer table showing the actual widths !
So folks , as one can find in life , all is not always what it appears to be .
The wheel calculators are only good for alloy wheel offsets .