Which tires are allowed, and what is the law?
Car tire sizes: meaning and calculation
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:
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.
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:
Table of tire size depending on the rim width
The following table shows the permissible tire sizes depending on the rim width:
Rim width
Tire width
Tire height
13 inch (ca. 33 cm)
155 to 175 mm
60 – 70%
14 inch (ca. 36 cm)
165 to 195 mm
60 – 70%
15 inch (ca. 38 cm)
175 to 215 mm
60 – 70%
16 inch (ca. 41 cm)
185 to 225 mm
60 – 70%
17 inch (ca. 43 cm)
205 to 245 mm
60 – 70%
18 inch (ca. 46 cm)
215 to 255 mm
60 – 70%
19 inch (ca. 48 cm)
225 to 275 mm
60 – 70%
20 inch (ca. 51 cm)
235 to 285 mm
60 – 70%
21 inch (ca. 53 cm)
245 to 295 mm
60 – 70%
22 inch (ca. 56 cm)
255 to 305 mm
60 – 70%
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