Quote:
Originally Posted by hwntime
Yes, from a look perspective they are stretched and it looks odd to me. I'll be going up and squaring to 235 or 245 on my next set. I personally don't like that stretched look and can only imagine how it would look on an 8.5". The contact patch area of the 225 is much more narrow than the wheel itself regardless of the wheel protection.
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Using this calculator that pulls information from a tyre database to give representative values for a given tyre size -
http://paws.kettering.edu/~amazzei/tire_calculator.html, the contact patch width at 2.5 bar (36 PSI) on a 225/40-18 tyre on a 7.5" rim will be about 200mm, 209mm on a 8.0" rim and 218mm on a 8.5' rim.
To get the balloon look of high profile tyre, a narrow rim will round the edges of the tread as the sidewall is pinched in by the narrower rim width. This will lead to the contact patch width being close to the equivalent of a narrower tyre, but with the disadvantage of being a heavier tyre. For example the contact patch width of a 225/40-18 tyre on a 7.5" rim will be about 200mm compared with 198mm for a 215/40-18 tyre on the same rim.
From a performance perspective at Auto-X events, it is always recommended to use a given tyre with the maximum certified rim width if that width is allowed for the car class, this ensures the flattest tread profile for best lateral grip. Using a tyre on the narrowest certified wheel, which is 7.5" for the 225/40-18 tyre, will lead to lower lateral grip than on the other certified sizes for Michelin Pilot Sport tyres of 8.0", 8.5" and 9.0". The "stretched" look (which is the way Michelin seems to have designed the Pilot Sport tyres to be mounted) gives better performance from a cornering perspective.