Since the question came up in another topic, I thought I'd give the quick 3 minute tutorial on sizing wheels to tires.
First you need to understand the difference between the terms section width and tread width. Section width is the distance from side wall to sidewall at the widest point. Tread width is just that, the width of the tread. The picture below illustrates the difference. The number specified in a tire's size, such as "335" in 335/35R17, is the tire's section width in millimeters. The tread width cannot be determined via this numbering system.
Each tire manufacturer will specify a range of wheel widths they approve their tire to be mounted on. As the aspect ratio of a tire decreases (shorter sidewalls) this range of wheels widths also decreases. But just because a certain wheel width falls within a manufacturer's specification does not mean that mounting that tire on that wheel makes good engineering sense for a desired result. The dimension for recommended wheel width will always vary from a width narrower than the tire's tread width, to a width wider than the tire's tread width.
Varying the width of a wheel in relation to the tread width of a tire affects the manner in which the sidewall supports the tread of the tire. A wheel that is narrower than the tread provides a softer ride, to the detriment of the slip angle required for a given cornering force. Steering response and road holding are worse, the effect is as though the sidewall were softer. A wheel that is wider than the tread has the opposite effect, a lower slip angle is required for a given cornering force at the expense of a stiffer ride. Steering response and roadholding are improved, the effect is though the sidewall were stiffer.
Because this is a sports car forum, it is safe to assume we are all interested in optimizing the traction and steering response of our car's chassis. The normal advice given when sizing wheels and tires for a sports car is that the tread width should never exceed the width of the wheel it is mounted on. It is not an issue of asthetics or safety, it is an issue of performance.
Look at the wheel & tire pairing of DeTomaso, Ferrari or any performance car manufacturer. DeTomaso tended to install tires on the widest wheel allowed by a tire manufacturer's specification. DeTomaso put 215/70R15 tires on a 15 x 8 wheel. Then Ford, in typical American auto manufacturer fashion, installed big old H60-15 Arrivas (255/60R15) on the same wheels (bling). Meanwhile, on the European GTS Panteras, DeTomaso moved the 15 x 8 wheels to the front and mounted sensible 225/50R15 Pirelli P7's on them. The 285 section P7's on the rear of the European GTS were mounted on 10" wide wheels.
Another example the Corvette guys will relate to. The C4 'Vette had the same size tires at all 4 corners, but the front wheels were 1/2" narrower than the rear wheels, in order to dial a small amount of understeer into the chassis.
cowboy from hell
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