There are a number of factors that affect a tyre’s performance. Some of these are integrated features and design traits while others are directly linked to the diameter and sidewall height of the tyre in relation to the vehicle it’s driving.
While many drivers acknowledge that different tyre tread designs are best suited to different terrains and driving conditions, what they don’t necessarily consider is the impact of tyre size and design on the rolling circumference – in other words, how these details affect travelling distances.
In this blog, we unpick everything you need to know about the rolling circumference of a tyre, why it matters and what the limits are with regards to safe and legal discrepancies.
What is the Rolling Circumference of a Tyre?
The rolling circumference of a tyre is the total distance travelled in a single, full tyre rotation.
It is directly impacted by the width and height / profile of the tyre, both of which play a part in how much ground the tyre covers per rotation.
Impact of Rolling Circumference on Safe Driving
Once the rolling circumference of a tyre has been identified, it is used to inform the speedometer of a vehicle – with this an essential part of safe driving.
A discrepancy of up to +1.5% and -2.5% from standard tyres is allowed before the results start to be seen as excessively skewed and therefore unsafe. Which leads to another question… why would there by any kind of discrepancy at all?
In short, the percentage discrepancy is based on the standard OEM tyre fitment and its rolling circumference. There are all sorts of reasons why a driver may choose to replace the OEM tyres with fitments in a slightly different size or with different features, however it’s crucial that they pay attention to this percentage point and make sure it doesn’t exceed or sit below the legal circumference rating.
Not only does it cause speedometer readings to become inaccurate, but it can also cause issues during MOT inspections as the rolling circumference can confuse integrated braking systems and sensors.
How to Measure Tyre Rolling Circumference
The rolling circumference of a tyre can be measured by placing a small marker on the tyre and then tracking a full rotation, before stopping the tyre and measuring the distance travelled.
Alternatively, there is a calculation that you can use to work out the rolling circumference, using the tyre sizing information for your fitment.
In this instance, C = π x (R + 2 x (H/W)/100 x W) – where R refers to the wheel rim diameter in mm, H refers to the sidewall height, and W refers to the width of the tyre.
For reference, the rolling circumference tends to be a couple of metres in total distance – a fact that should help you check your own calculation!
Should I Always Buy Tyres in a Standard Size?
If you’re on the hunt for new or replacement tyres for your vehicle, you might be wondering what size to choose and whether you have to stick with the same size as the old tyres.
It really depends on how you use your vehicle, your local surroundings and terrain, and your ideal aesthetic. While you do not have to stick with the exact tyre size, drivers need to be aware of the implications of a different size on rolling circumference and therefore on speedometer measurements and vehicle safety – alongside other factors.
If you prefer tyres with a higher profile height or think wider tyres will support better traction on wet roads, then make sure to consider this when measuring your rolling distances.
For more advice like this, check out our other blogs and articles in the Knowledge Hub.
