Tyre contact patch is determined by the tyre design. Go to the Michelin website for an explanation. It has very little to do with operating pressure or sidewall stiffness, or any other structural factor.

The weight of the car determines tyre pressure. Since the weight doesn't vary by very much, then neither should the tyre pressure. Rubber has a given coefficient of friction against a road surface, and this determines grip. If a car of a given weight needs a certain "feel" or "grip" then the designer looks at a manufacturer's catalogue, looks at the models available, then chooses a tyre in combination with a given wheel size and rim width that offers the largest/most economical/whatever contact patch possible for his design. He then looks at the all-up weight of his car, looks up the manufacturer's recommended tyre pressure for that tyre, does testing, tweaks it until it feels OK, and then repeats with many different makes of tyre of the same design characteristics. He then specifies a pressure that makes all the tyres of the given design size work properly for that application.

When the car is rolling, then there is a measure of force applied downward by the atmosphere acting on the bodywork, hence the effective weight of the vehicle increases. Hence, for constant high speed motoring, you increase tyre pressures.

Tyre wear and handling are affected when you allow the pressures to very by more than 10% above or below the manufacturer's (car, that is) recommended pressure. However, far more rapid wear is usually caused by your wheel alignment being incorrect, or how hard you corner (front wheel drive vehicles, mainly). If you kept your tyres 25% over pressure for a year, you may begin to notice crown wear. If your alignment was out by a degree or so for just a month or two you would see significant shoulder wear. Of the two, high tyre pressures would as like as not kill you in the first wet weather heavy braking you encounter.

Tyre weight, sidewall stiffness, carcass construction and all the rest do not generally affect the handlng characteristics of a tyre - otherwise tyres would vary in behaviour between manafacturers or be dangerous to use, and manufacturers would rapidly go out of business. The tyre's structural behaviour is effectively that of a three-dimensional spring, whose characteristics are largely determined by the volume (and therefore mass) of air they contain.

The number of plies in a crown or in the sidewall usually determines the tyre's ability to resist deformation on hard rim impacts, and resist penetration leading to punctures. A rubber-only crown, for instance, would puncture rapidly and would as likely or not burst instantly: a ply protected crown will resist a puncture, and even if punched clean through the ply layers, the plies would prevent the crown rupturing from internal pressure. Instead, the tyre deflates gracefully giving you more time to come to a halt. And before you start saying that the tyre will come off the rim if this happens, no it probably won't because the rim is designed to stop this (J type and K type are, anyway) and the steel bead around the foot of the tyre wall is re-inforced, usually welded steel made to a close tolerances and high quality standards.

Don't play around with tyre pressures in a misguided attempt to make the car "handle better" or some such. It will bite you back. You need to look at your suspension as well, not just the tyre. Don't forget, one of the first things the coppers do at a major accident site is measure the tyre track lengths, look up the coefficient of friction for that piece of road surface, and then take the tyre pressures of the crashed vehicles. It all goes into the accident report: some insurance companies will invalidate your insurance cover in the event that there is evidence of incorrect tyre pressures (no joke, this).

Moral of the story:

- Stick to the CAR manufacturer's recommended tyre pressures, for all speed conditions
- check your tyre condition and pressures regularly
- don't play about with safety critical equipment, especially if your family or other passengers are travelling in the car.