When it comes to buying new tyres and rims, you should always check that they are the correct size authorised for your vehicle, or else your vehicle will no longer be compliant with the highway code, thus jeopardising the conformity of your car. The rules in this respect often lead to misunderstanding, and I’m about to explain why.
You will find the right combination of rims and tyres on your vehicle's Certificate of Conformity (COC), in the entries on lines 35 Pneumatique des roues (‘Tyres’) and 52 Remarques (‘Notes’). There may be several approved rim/tyre combinations listed here, for both summer and winter tyres. The specific dimensions stated relate, on the one hand, to safety and driving behaviour, with steering and wheel alignment, the braking system and mileometer readings combined with the various braking aids the car might have, such as ABS and ESP, just some of the parameters that will need to be adapted to the wheel dimensions stated. On the other hand, the tyre dimensions predetermined by the vehicle manufacturer are calculated based on fuel consumption and therefore also based on important CO2 emission approval values.
The Certificate of Conformity also states the dimensions of the rims; the diameter and width of the rim are stated in inches and the offset, that is the distance in mm between the mounting surface and the geometric centre of the rim, designated by the letters ET. The smaller the number that follows ET, the further outwards the rim is offset on the vehicle and the wider the vehicle’s track. This simple example makes it easier to understand how a different rim size will have a significant impact on driving behaviour and tyre wear.
Tyre dimensions including the tyre width in mm, the tyre sidewall height as a percentage of the width, and the diameter in inches, are also stated in the COC documents, along with additional information on the load capacity index (maximum load per wheel), the speed symbol (maximum permissible tyre speed) and whether the rubber is suitable for winter use, as indicated by the M&S symbol. Unlike in the past, the dimensions of the latter are now often identical to those of summer tyres, precisely for the reasons outlined above.
In the case of specific types of car, such as sports cars, the manufacturer will often approve only one type of tyre that has been manufactured specifically for that vehicle in conjunction with the tyre manufacturer. In the event of a difference in size between the front and rear axles, this information will also be stated on the COC. Should you wish to fit snow chains, it is vital that they match the tyre dimensions.
Should you still wish to fit your vehicle with other wheels, you will need a national approval certificate (ABE) from the TÜV. This document certifies that the selected wheels are compatible with the vehicle. You can then have the wheel/tyre combination registered by the SNCA on the vehicle registration document.
If you do not have this national approval certificate, I recommend that you have a separate vehicle acceptance test carried out at the ACL-Diagnostic Center in conjunction with the TÜV. You can make an appointment by calling +352 450045-6007.