I just want to try and clarify a few misconceptions about car tires and friction. When you increase the psi of the car's tires the mpg increase is not from less friction with the road, it is from less deformation of the tyre.
Rolling resistance and road friction are two separate things. As the tyre comes in contact with the road a section deforms to become flat against the road. As the tyre rolls away the rubber returns to its original shape. This compressing and expanding of the tyre creates heat which wastes energy. This can be reduced by higher pressures. The other part of rolling resistance is adhesion forces where the tyre sticks to the road and is then peeled off. Adhesion is more directly related to grip but is more a function of the tyre compound than the pressure. The reason wider tyres are reported to have more grip is because the wider contact patch gives more surface area for the tyre to cool down in high performance situations, i.e. situations where grip is lost due to temperature. This seems to be the source of the myth that a bigger contact area gives greater friction. The maximum grip of tyres before skidding is proportional the coefficient of static friction (i.e. for rubber on concrete it is 1.0) times the reaction force in newtons. A quote from my university Physics textbook "Physics for Scientists and Engineers" by Serway, Jewett:
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The coefficients of friction are nearly independent of the area of contact between the surfaces. We might expect that placing an object on the side having the most area would increase the friction force. While this provides more points in contact... the weight of the object is spread out over a larger area so the individual points are not pressed so tightly together. These effects approximately compensate for each other, so that the friction force is independent of the area.
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I'm not trying to sound like an expert or anything I just wanted to clear up some myths being thrown around.