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Thank you for your detailed response, but it seems that my original point might have been lost. Let me explain a bit more clearly.
The reason we do not recommend a 265mm tire on a 9” wheel is due to the lack of sidewall support when compared to a 245mm or 255mm tire on a 9” wheel. That is not to say that a 265mm tire cannot work on a 9” wheel or is dangerous or unsafe, but it can and will impact the performance, handling characteristics (feedback to the driver) and wear patterns of the tire in question. When you consider the fact that many extreme summer, r-comp and semi-slick track oriented compounds notoriously run wider than your average 265 street tire, the lack of sidewall support from a 9” wheel is amplified and becomes increasingly more apparent to the driver - especially upon initial turn-in and throughout a corner when the weight of the vehicle is transferred and the tire sidewalls are experiencing extreme loads.
When fitting a wheel and tire package to any car, many individuals believe that wide tires unequivocally result in more performance. That is not always the case. A crucial variable to take into consideration is maximizing sidewall support on whatever size tire you plan on using. When the sidewall of a tire is supported correctly, the tire can perform much more linearly across its traction range. If the sidewall is not well supported, the tire sidewall can flex and fold over during turn-in and throughout corners. The sidewall is not designed to withstand the same adhesion requirements as the main treadblock of a tire, and this folding over can result in less traction and undesired understeer/oversteer given the situation.
Generally speaking, the consensus of many racers is that in order to extract the most performance out of a given compound and to maximize the tires contact patch, you’ll want to use a wheel width that is at the wider end of a tire manufacturer's recommended wheel width spectrum. Tire sidewall construction varies from compound to compound, so some tire models are more sensitive to this than others.
There are always exceptions to the rule. Many racers are bound to class rules, for example, SCCA street class mandates that you run the stock wheel widths of a given vehicle with an offset that is +/- 7mm. In these instances, while drivers cannot run wider wheel widths, they will certainly maximize tire width to gain as much grip as possible.
At the end of the day, hard core track enthusaists and racers are always looking for ways of getting an edge over “the competition”. There are many decisions and incremental improvements that can be made which directly increase performance and/or instill more confidence in the driver, all resulting in superior lap times.
I do hope that this provided clarity to my previous posts.
- Max
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