What they’re made of they are not really round. There is a flat spot on the bottom where the tire meets the road. This flat spot is called the contact patch. This helps the tire have more space on the ground so that the square inch area touching the ground is higher. By doing this, You don’t need as much pressure in the tire. Tires usually only need 30 pounds per square inch. You can figure out how much four tires can hold by adding the square inches touching the ground and multiplying it by four (all tires). Then you must add in the area pressure of all the tires. You find this by finding the area of the tire in square inches and multiplying that by 30. Related equationsTire manufacturers sometimes publish a coefficient of rolling friction (CRF) for their tires. You can use this number to calculate how much force it takes to push a tire down the road. The CRF has nothing to do with how much traction the tire has; it is used to calculate the amount of drag or rolling resistance caused by the tires. The CRF is just like any other coefficient of friction: The force required to overcome the friction is equal to the CRF multiplied by the weight on the tire. This table lists typical CRFs for several different types of wheels. Tire TypeCoefficient of Rolling FrictionLow-Rolling Resistance Car Tire0.006-0.01Ordinary Car Tire0.015Truck Tire0.006-0.01Train Wheel0.001Let's figure out how much force a typical car might use to push its tires down the road. Let's say our car weighs 4,000 pounds (1814.369 kg), and the tires have a CRF of 0.015. The force is equal to 4,000 x 0.015, which equals 60 pounds (27.215 kg). Now let's figure out how much power that is. If you've read the How Stuff Works article How Force, Torque, Power and Energy Work, you know that power is equal to force times speed. So the amount of power used by the tires depends on how fast the car is going. At 75 mph (120.7 kph), the tires are using 12 horsepower, and at ...
