Tires like people come in all shapes and sizes, from monster trucks to lawn mowers they all take some sort of tire. There can be so many choices when buying tires it can be confusing. Tires are not like a wheel hub assembly or wheel bearing assembly there are tons that can fit on the same car. Some tires are able to fit under a car but are too large to allow the car to turn and others are too small making all the gauges in the car inaccurate. Not to mention tires that are too small will lower the performance of the car.
There are numbers that are listed on the side of a tire that will indicate the diameter and thickness. These numbers are listed in a very specific order which as an example would be something like a 205/40/85R. This code can be broken down by section the 205 is the width of the tire. The 40 refers to the height of the sidewall and the 85R will refer to the compound of the tire and the speed rating.
The width of the tire will decide the contact patch on the road, this is the area where the tire is actually resting on the road. This is also the only area where the horsepower of the car is actually transferred to the pavement. The height of the sidewall will dictated the comfort of the ride since the sidewall is the area that absorbs the small bumps in the road. The sidewall also protects the rims from dents due to large impacts. The speed rating literally dictates the speed at which the tires will begin to fall apart. Exceeding the speed rating of any tire is dangerous and potentially fatal due to the chance of failure and the resulting accident. The compound rating will tell the stickiness of the rubber used, while a stickier tire will give more grip it will wear out much faster than other tires.
Selecting the correct tire is essential to the ride and handling quality of the car and is best left to the professionals. If you are going to buy your own tired be sure to study up on the suggested tires for your individual ride.