The S1 Exige has the following tire specs:
F: 195/50 R16 on 7 inch wide rim
R: 225/45 R17 on 8.5 inch wide rim
The S2 Exige has the exact same tire sizes, but on 6.5 and 7.5 inch wheels front and rear.
This got me thinking about ratios of tire and wheel sizes. I found an article here
that describes the tire selection process using Tire Rack's tread width against the wheel width. The summary of the article is to pick a tire that has a tread width equal to or 0.5" narrower than the rim width. However, the S2 Exige's OEM tire tread width (front and rear) are both more than an inch wider than the rim widths. This is common as well. My Cayman tires look like balloons on the wheels despite being the standard sizes.
If you think about the cross-section of the tire on the rim, it looks like a trapezoid and the sidewall angle can be calculated (using tire width, wheel width, and tire profile). Tread width was ignored as specific tires were not taken into account. Trying to find a trend with side wall angles, I did some calculations for OEM wheel/tire sizes of mid-engine cars (data in degrees). Note that all of these tires are wider than the wheels that they are mounted upon.
Cayman S (987):
9.601 Front <-- This is why they look like balloons I suppose.
Ford GT (1st Gen):
??? There doesn't seem to be any trend here. I was under the impression that a more vertical sidewall (0 deg) is better for traction purposes. A piece of paper can be bent into the shape of a tire cross-section and the bottom becomes flatter as the side walls become more vertical. That is what the article above would suggest as well.
Now on to the questions! Why do OEMs tend to not follow this rule and why does there seem to not be a trend? If you have a set of wheels already, it would be best to pick the widest tires possible, right? Should one think about these calculations to select wheels and tires? Is there some sort of general knowledge that I should know about this relationship? Sorry for the long post but I hope to learn from you guys!