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Marking car tires is a daunting task for most car enthusiasts, as many are not familiar with the markings on the outside of the rubber walls.

Meanwhile, the wheel diameter is a parameter with which you can select tires for a car that meet the technical characteristics of the vehicle. These parameters are important when choosing summer or winter wheels, and during scheduled tire changes in order to improve certain characteristics of the car.

Before choosing the diameter of a car wheel, it is important to understand what the profile of the tire and the offset of the disks are in order to obtain the recommended values ​​for installation. To ensure the best handling, stability of the car on the road and high cross-country ability, it is necessary to select parameters that correspond to the technical regulations of the car, operating conditions. Below you will find information on tire markings and design, and how you can select tires using a tire calculator.

What is diameter and radius

Despite the large amount of information, the designations on the tires, which are indicated by the manufacturer, many drivers confuse the radius of the wheels and the diameter, or even understand one parameter under different concepts. In practice, rubber manufacturers do not use the "radius" parameter as it refers to the distance from the center of the tire to its edge. This wheel measurement is not used.

Tire manufacturers measure rims in inches, so the tire design and rims are nothing more than diameter. It means the distance between the opposite edges of the tires. The circumference is measured along a line through the center of the wheel. This parameter is used more often, because it is easy to measure, and also shows the optimal values ​​that allow you to accurately select a wheel or tires for any car. Other indicators stand out:

Landing diameter. With the help of these values, the admissible parameter of the rims for the installation of rubber is selected. In order to avoid problems during the installation process, the bore diameter must be of the permitted size and exactly fit the disc. A few centimeters of difference will prevent the wheel from being fitted due to possible tire damage. Outside diameter. In addition to the inner diameter, the outer diameter is also measured in tires. It represents the difference between opposite points located on the outer perimeter of the wheels. The parameter is measured by manufacturers according to a certain formula. The accuracy of the value determines how accurate the odometer reading will be.

It is worth noting that the letter R has nothing to do with radius, as many people think. It denotes the type of tire - there are radial and bias models. The first ones, which are marked R, differ in the structure of the frame. Here the cord is pulled between the beads without overlapping the threads. The elastic wheel shell is covered with a special belt made of a strong cord that does not deform under load. This technology makes the construction of the tires durable and improves the grip of the rubber with the road.

Bias tires have a similar tread, identical radius, but differ in the connection of threads. Unlike models with the R marking, the threads in the structure are located diagonally between the sides. The belt layer, which holds and stabilizes the tires, is less stable during rolling, which leads to faster tire wear. What are the sizes

Manufacturers offer tire models with different parameters and radius. Depending on the type of car, drivers can choose the diameter of a passenger wheel from 13 to 20, as well as the width of the rim, which is measured in inches. There is a significant difference between the models. They differ in profile height in centimeters, speed qualities, wear resistance. Since it is not easy to measure by yourself which wheels will fit a car, many automakers indicate the recommended wheel parameters on the car door pillar.

To install the wheels correctly, tires must be selected based on their size. This parameter is applied to the rubber sidewall in the following form: 225/45 / R 18 84T. You can decipher the marking in the following way:

  • 225 - rubber profile width;
  • 45 - profile height, calculated in relation to the tire width as a percentage;
  • R 18 - the number indicates the diameter of the tire, the letter indicates the radial type of rubber;
  • 84 - parameter of the index and the load factor that the wheel can withstand;
  • T is the permissible speed index at which the car can be driven.


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