Shopping for replacement tyres can be stressful when you’re exploring new options. If you want to experiment with different tyre sizes, then it helps to know what advantages and disadvantages you’ll be working with.

Your replacement tyres can be used to change your vehicle’s aesthetic appeal, improve your road safety, and improve your fuel economy. Where, though, is the best place to start?

Choosing your replacement tyres

Good news! If you’re dissatisfied with your car tyres, you can invest in a set that is structurally different from the ones that came with your vehicle. Most automotive manufacturers ensure that your car can work with wheels that have variable outer diameters. However, you’re constrained to tyres that have an outer diameter within two per cent of your original tyre’s.

If you do want to invest in tyres that are significantly larger or smaller than the ones that originally came with your car, then you’re going to need to change the size of your rims or otherwise preserve the combined diameter the manufacturers originally suggested for your vehicle.

While customising your ride is part of the run of owning a car, remember that automotive manufacturers only guarantee tyre traction for tyres within your vehicle’s approved tyre size. Any fluctuation means that you’re on your own. To determine your vehicle’s recommended tyre size and working pressure, take a peek at the tables by your fuel cap or by your door sills.

Larger wheel diameter: the pros and cons

Interested in experimenting with a set of tyres that are larger than your current set? There are advantages and disadvantages to going big. Some of the perks of working with a larger tyre include:

  • Increased grip: The grip pretty much depends on the tyre height and width. The wider your tyre is, the more quickly it will come to a stop on dry roads.
  • Turn reaction time: When you have a larger rim diameter to work with, your car will be more stable when it goes around corners, and you’ll be able to make tighter turns.
  • The size of your brake discs: When your tyres have a larger diameter, you’re able to invest in larger car brake discs as well as larger callipers. As a result, your car will handle more easily and stop more safely under extreme circumstances.
  • Appearance: Finally, if you’re interested in improving your car’s aesthetic appeal, then larger tyres will serve you well.

A mechanic in a workshop changing tyres

As mentioned, though, larger tyres do have their downsides. When you invest in a larger set of tyres, you’ll have to deal with:

  • Increased cost: Above all, larger tyres are going to cost more than tyres that fit within your car’s recommended sizes. Why? Because to create wider tyres, manufacturers have to resource better rubber and develop more complex tread. These tyres also take longer to manufacture, meaning the manufacturer will have spent more money on man hours and fuel. With this in mind, have a budget prepared in advance so you’re not surprised by the cost that comes your way.
  • Greater chance of rim and tyre damage: Even though the diameter of your tyres is going to be larger than it used to be, your tyres are going to be shorter than before. As a result, they’ll be at a higher risk of sidewall damage when you drive over potholes or other obstacles in the road.
  • A less comfortable ride: The more surface area your tyres have, the more obstacles they’re going to be able to conquer. However, these tyres won’t always ensure a comfortable ride. Wider tyres have more rigid sidewalls, meaning there’s less flex and give to your ride. This is great for off-roaders, but not so great for someone commuting in an urban area.
  • Aquaplaning risk: While the tread on your tyres will protect you from roadway obstacles, they’ll fill up with water more quickly on wet roads – and there’ll be fewer opportunities for your tyres to shed that excess. As a result, you have a greater risk of aquaplaning, or losing control of your car while driving in the rain or snow. Of course, many manufacturers will experiment with asymmetrical tread to try and compensate, but wider tyres will never have the same ability to disperse water as tyres that are taller and narrower.
  • Increased noise: Larger tyres are also going to be noisier than smaller tyres. Why? Because the larger width exposes your tyre to more of the road, meaning that it will come into contact with more roadway obstacles and, thus, generate more tyre noise.
  • Reduced grip on dirty roads. When the tyre width is increased, there is nearly always an increase in the braking distance on sandy or dirty roads.

Replacement tyres graphic

The maths of tyre replacement

If you’re not interested in a larger set of tyres, you can just as easily invest in a set that’s a little smaller than the ones you already have. Ideally, you’ll want to keep your tyres within the same two per cent range that you would when shopping for larger tyres. It is not recommended that you reduce your tyre size below 205 mm, as the concentrated tyre pressure won’t be safe to maintain.

Note, too, that this metric for error was impacted by the European Tyre and Rim Technical Organisation in July 2003. In accordance with the tyre regulations dictated by this organisation, the diameter of your tyres, including the rims, cannot deviate more than or less than three per cent from manufacturer recommended size.

Where can you start looking, then, if you want to replace your tyres?

It’s best to start your search by calculating your current outer diameter as well as the diameter that you’re wanting to adjust towards. To do so, you can use the formula below:

replacement tyres formula

If you’re working with 185/60 R15 tyres, for example, you’ll find that the aforementioned calculation puts your potential outer diameter flux between 598.78 mm and 620.165 mm. This range, however, comes from calculations made prior to the institutions of the ETRTO.

Going with the ETRTO’s standards, you’ll find that the acceptable variation range for 185/60 R15 tyres falls between 596.34 mm and 609.66 mm.

Your options for replacement tyres that are larger or smaller than those your manufacturer has recommended are going to be limited then. That doesn’t mean however, there aren’t options available to you. But still, for the safest ride it’s often best to commit to a set of tyres that fits within the outer diameter the professionals have recommended.

Calculating possible replacements for size 185/60R15 tyres:

table of possible replacements for 185/60R15 tyres

When to shrink your rims

If you do want to shrink out your rims, consider leaving your replacement job for the winter months. Winter tyres tend to require smaller rims, and you’ll be able to more readily fit rain-and-snow tyres to your smaller rims. 

Before you invest, however, make sure that your brake discs and callipers aren’t too large to work with a reduced rim set. While it’s possible to modify these essentials to fit smaller rims, the work is going to target your wallet – and going back on the job is a pain.

Above all else, consider the impact smaller rims are going to have on your car. Rims not approved by your manufacturer can improve your ride, so long as they’re paired with the right set of tyres. Don’t get too adventurous without consulting the professionals at your local garage.

A mechanic installing a car tyre

Smaller Diameter Versus Normal Diameter: What’s The Difference?

Tyres with a smaller diameter will be taller but narrower than your traditional tyres. Smaller tyres like these are ideal for winter, because, while they have less surface area, they’ll be less prone to damage when the roads are slicker. Smaller tyres are less likely to aquaplane and have less surface area to damage. Smaller tyres are also going to be a little more affordable than your standard tyres or larger sets.

That said, you won’t be able to customise your brake pads as readily when working with tyres with smaller diameters. Likewise, it’s possible that your response time will suffer as you adjust to your smaller diameter.

Playing with the size of your tyres’ diameters can bring you improved safety and style. Try going up an inch in size, or see what variation your current outer diameter allows for. If nothing else, it’ll be fun to consider the opportunities your replacement tyre set may allow you.