Taylor Made: Mechanics of tires
One of the largest and most well known contributions to the aerodynamic performance of a car comes from its body shape and structure. However, characteristics of its wheels and tires can make a significant contribution as well.
The more a tire is inflated, the less rolling resistance it has. This happens because the contact area of the tire (the part of the tire in contact with the ground) decreases as the tire becomes more round (i.e. more stretched out) from increased air pressure. Higher pressure also means that the rubber is stiffer, leading to less loss of energy through flexing of the tire walls. Since decreased contact area leads to less traction, the relationship between air pressure and traction is another important design consideration.
Bike tires are usually inflated between at least 40-60 psi for average street bikes and higher for racing bikes. In contrast, car tires are often kept between 30-35 psi. In the case of car design, traction is often more heavily weighted (house of quality, anyone?) than rolling resistance because cars have a great source of mechanical power that can compensate for increased resistance and energy loss at the tires. For bikes the rider provides the mechanical power, making conservation of energy more important.
Tires can be filled with regular air or other blends of gas such as nitrogen. No matter what gas they’re filled with, they will lose pressure over time due to the gaseous particles permeating the rubber. Since packed air provides strength, it is actually considered best to keep tires at full psi. The increased structure prevents blowout on the road, which is often caused by underinflation. In cases of failure due to overinflation, it is more common that the wheel fails under the pressure than the tire wall ruptures.
With compact spare tires, also called donuts, different rules apply. These tires are much smaller in diameter than the car’s normal tires and are only meant to be used for moving the car a short distance. Similar to bike tires, they’re usually inflated to around 60 psi. It is recommended that a car driving on a compact spare does not exceed 50 mph or travel long distances.
Cars today have differentials, i.e. gear trains, that allow the wheels to travel at the same speed while rotating at different rates. This is necessary because in a turn the outer wheel needs to spin faster in order to keep up with the inner wheel. The differential also allows the car to drive on wheels of different diameters, but since it is not meant for this purpose, prolonged driving can potentially wear it out.
Fun fact: In order to withstand extreme force during landing, F-16 tires are pumped to around 320 psi for max strength.