the "width" size of the tyre has nothing to do with the tyre width at the rim, its only the width of the tyre. and even then the "width" size of the tyre is not necessarily the same if you measure the contact area of the tyre to the road.
if you look at road tyre sizes and racing tyre sizes the only difference in theory should be the height.
road size 225/40-18
mm width / percent height - rim size in inches
width: 225mm
height: 90mm equivalent to 637mm complete tyre height
rim size: 18"
racing size 210/650-18
mm width / mm height complete tyre - rim size in inches
width: 210mm
height: 96mm equivalent to 650mm complete tyre height
rim size: 18"
so the 210/650-18 is only a little bit higher then your 235/40-18 tyres, and some what higher then 225/40-18 tyres. so you get more topspeed in top gear/max rev.
not saying that you have the wrong size or anything only trying to explain the theory and numbers on paper.
Gulfstream is right you are wrong on this occasion.
What you say may logically make some sort of sense but race tyres are measured or sized differently overall, not sure how or why but i know they are as i have looked into this in a LOT of depth before. As gulfstream said the nearest race tyre to a 225/40/18 road tyre is a 210/650/18 tyre. Dunlop used to do this tyre in 210/640/18 which was fractionally closer but is no longer made.