Tips and advice to buy the right tyres - Can I change my tyre size?
Center

Can I change my tyre size?

Buy the right tyres Peter

You want to buy tyres in a size other than your car’s original equipment, for either "tuning" purposes or to save money. Just remember that strict criteria must be met otherwise you take the risk of infringing regulations or risking your life. On this matter, Rezulteo’s position is quite simple: Use manufacturer approved sizes.

Equivalent sizes for front tyres   Equivalent sizes for front tyres - Copyright © : iStockphoto.com/rezulteo.com

This question is regularly asked on forums or in our expert section : Am I allowed to change from this tyre size to another one? The reasons vary. For some, it is all about giving their car a sporty look by fitting bigger wheel rims. For others, it’s more a question of saving money by opting for a cheaper size than the tyre originally fitted.  Whatever the reason, changing tyre size is not a trivial matter. It has technical and regulatory implications that cannot be ignored.

 

Only manufacturer approved sizes are valid

It is the car manufacturers, and they alone that determine the tyres sizes suitable for a vehicle.  Based on studies and tests, they guarantee a maximum level of performance and safety. Any deviation from these sizes means running the risk of changing how your car handles, therefore your safety. Also, at the very least, you risk a fine if you are stopped by the police or at an MOT re-test and at worst, you may not to be covered by your insurance in the event of an accident.

 

Rules for tyre size equivalents

What criteria must be met if you want to change your tyre size? There are three such criteria:

  • The load index and speed index of the new size must be the same or higher than the original tyre.
  • The outer tyre diameter should be essentially the same as the original tyre to preserve the vehicle’s characteristics, and in particular, the accuracy of the speedometer and driving aid settings (ABS, ESP…). 
  • There must be enough clearance inside the wheel housing to accommodate the new size (do not forget to additional clearance if you want to fit chains in winter).

Remember, theoretical tyre size conversion charts exist that meet the criteria defined by ERTRO, the European Tyre and Rim Technical Organisation. These charts are by no means a guide to transform or “tune” your car.

 

How to find approved tyre sizes?

On this matter, Rezulteo’s position is quite simple: Use manufacturer approved sizes! But to find them …

  • Tyre sizes approved for your car are usually marked on the driver’s door pillar or in the vehicle’s technical documentation and/or on the vehicle’s registration documents.
  • They can also be found by using the tyre selector on Rezulteo’s home page.
  • Finally, you can also ask an MOT test centres that has access to the official database.

 

Display of Rezulteo’s tyre selector results

Display of Rezulteo’s tyre selector resultsCopyright © : rezulteo

For a BMW 3-Series, the Rezulteo tyre selector proposes 6 approved dimensions including 3 staggered wheels for 4 rim diameters.  

 

Wide tyres: Advantages and disadvantages

Wider tyres increase the contact patch and therefore deliver more grip on dry roads. They also provide a fast steering response. However, they may have a negative impact on wet braking (less water is drained), fuel consumption (higher rolling resistance) and comfort, reduced sidewalls do not absorb road irregularities as well.