Tire patch - same size for most tire widths
 

I was at a Porsche club event a few weeks ago. During the ground school portion, they talked about the size of the contact patch a tire has on the road.

To my surprise, they made the point that changing tire sizes and widths tend to change the shape of the patch more than the size (square inches). The car's weight and tire pressure largely determine the size of the contact patch.

A narrower rim produces a more longitudinal patch (lengthwise, with the car), making it better for snow tires because there is a narrower surface rolling forward over the snow.

A wider rim produces a more transverse patch (across the car), making it better for dry, performance driving.

Hm-m-m. I had always assumed that wider tires gave you more contact with the road, but apparently not.

This is something that I point out to a lot of people that have to have the widest tires possible, you know, for performance. The fact is, the amount of rubber on the road is pretty much determined by the two factors you mention - weight and tire pressure. Yes, there are some variances due to tire construction and rim width, but it's negligible.

It's true that a wider tire (which gives a short and wide contact patch), on a properly-sized rim, will give you more stability while cornering, but that's another discussion altogether. Conversely, wider tires do nothing for straight-line traction (and may hurt it). Much better to change the compound for better stick.