Which tires are allowed, and what is the law?

Tires are an important component of any car. They provide the connection between the vehicle and the road and are responsible for the safety and driving performance of the car. Therefore, it is essential that the tires meet the legal requirements.

Because not all tires are approved for all vehicles. If you mount the wrong tires, you risk not only your driving safety, but also your insurance coverage and a fine. This post will address issues related to tires, such as those you’re allowed to use, how to determine and calculate the size of the tire, and the differences between various sizes of tires.

CALCULATION AND MEANING OF TIRE SIZES

Tire size has a major influence on the driving characteristics of the car. A wider tire width provides more driving stability and grip, but also higher rolling resistance. A higher tire height provides more comfort and driving feel, but also a lower top speed.

The tire size can be calculated with a simple formula:

• Tire diameter: 2 * tire width * tire height / 100
• Tire circumference: Tire diameter * Pi

The tire size consists of the following sizes, which you can find on the sidewall of the tire in a combination of numbers and letters, for example: 205/55 R 16 91 V.

• The tire width in millimeters (e.g., 205 mm) indicates how wide the tire is at its widest point. The width varies depending on the rim width on which the tire is mounted.
• The height-to-width ratio in percent (e.g., 55%) indicates how high the sidewall of the tire is in relation to its width.
• The construction type of the tire (e.g., R for radial) indicates how the carcass of the tire is constructed. Most modern passenger car tires are radial tires (R), in which the carcass threads run radially to the direction of travel.
• The rim diameter in inches (e.g., 16 inches, ca. 41 cm) indicates the diameter of the rim on which the tire can be mounted.
• The load capacity index or load index (e.g., 91) indicates the maximum weight the tire can carry.
• The speed index (e.g., V) indicates the maximum speed the tire can withstand.

PERMISSIBLE DEVIATIONS FROM THE TIRE SIZE

In some cases, deviations from the tire size specified in the vehicle registration document are also permitted. However, these deviations must be approved by an expert.

The permissible tire size for a car is specified in the vehicle registration document. The vehicle registration certificate also contains information on the maximum permissible speeds and axle loads.

The tire size must not be larger than the size specified in the vehicle registration document. This also applies to the tire width, tire height and tire diameter. However, the tire size may be smaller than the size specified in the vehicle registration document. In this case, however, the maximum permissible speed and the axle load must be reduced.

Permissible deviations from the tire size are:

• +/- 3% for the tire width
• +/- 5% for tire height
• +/- 2% for tire diameter

However, if you want to use other tire sizes, you have other options. In the so-called CoC document (Certificate of Conformity), which is part of the vehicle documents, you will find all wheel and tire combinations that are approved for your car under numbers 32 and 50. You do not have to have these entered in your vehicle documents as long as you have the CoC document with you.

Another option is to carry out a tested and certified conversion. In this case, you can have tire dimensions that deviate from the specifications in the vehicle documents fitted if they are approved by an expert and entered in the vehicle documents. However, you must bear in mind that further modifications to the vehicle may be required, such as to the bodywork or the brake system.

What are the advantages and disadvantages of different tire sizes?

The choice of tire size not only affects the appearance of the car, but also the driving behavior, fuel consumption and wear. There are various advantages and disadvantages to consider:

• Wider tires have a larger contact area and thus better grip on the road. This improves cornering stability and braking behavior. However, wider tires also have higher rolling resistance and thus higher fuel consumption. In addition, wider tires are more susceptible to aquaplaning and crosswinds.
• Flatter tires have a lower sidewall height and thus greater stiffness. This improves steering precision and handling. However, flatter tires are also less comfortable and generate more noise. Furthermore, flatter tires are more susceptible to damage from curbs or potholes.
• Larger rims have a larger diameter and thus lower ground clearance. This improves the appearance and aerodynamics of the car. However, larger rims also have a higher weight and thus higher fuel consumption. In addition, larger rims are pricier to purchase and maintain.

Table of tire size depending on the rim width

The following table shows the permissible tire sizes depending on the rim width: