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PSI is pounds per square inch.
Literally, you can take the weight of the car (in this case 500 pounds in our example), and divide by the PSI (50 in my example), and get the solution that 10 square inches is required to hold the car up.
Because of Newton's "equal and opposite reaction" , if there is 50 psi pushing down, there is 50 psi pushing up.
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We have all seen where if you take a tire off the rim, and stand it up on its tread, the tire holds it self up. therefore, the sidewalls DO hold the tire (and the car) up somewhat.
If we stand on the inside bead of the tire, the tire does squish downward somewhat.
If we take a normal car, let the air completely out of the tire, the tire goes flat, and the rim rests on the ground).
Therefore, the sidewalls of the tire DO hold the car somewhat, but not nearly enough to hold the weight of the car up.
Ok. Back to MPG. Thanks to someone else's post, the majority of rolling resistance of a car comes from tire deflection.
Tire deflection is shown by looking at any radial tire, and the bottom is pooched out, the rest of the tire is "straighter".
As the car rolls, the a particular spot on the sidewall "pooches" then straightens. This flexing of the tire sucks up energy and is known as Hysteresis by the geeky types.
Suffice it to say how much energy it takes to flex the tire depends on the weight of the vehicle, the design of the tire, and how MUCH the tire is pooching.
Most of us cannot change the weight of the vehicles signficantly. Radial tires flex easier then bias plies, and new tire technology flexes easier.
one thing EASY to change is how MUCH the tire flexes. We do this by keeping our tires inflated at maximum recommended amounts - this means the tire pooches less, and we get better mileage.
As a side note, when a tire gets low in air pressure, the tire pooches a LOT which builds heat in the tire which is why a tire gets really frigging hot and can actually catch fire if you drive down the freeway on a severely underinflated tire.
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