19/09/2016
What to bear in mind when choosing replacement tyres
Vehicle manufacturers do permit the outer tyre diameter to be varied, but only within a range lower than 2%. Therefore, if fitting wheel rims with a larger diameter, it is necessary to reduce the profile of the tyres to compensate and maintain the overall combined diameter.
Before making any changes, remember that, in most cases, the use of officially approved tyre sizes is the only guarantee of retaining the manufacturer's specified traction parameters.
Advantages of increasing wheel diameter
There are many advantages to using an increased wheel diameter. These are:
Increased grip. The degree of grip improvement depends, to a large extent, on the amount of change in the tyre height and width. Also, increasing the width further shortens the braking distance on dry roads.
Improved reaction and behaviour on turns. An increased rim diameter, combined with a smaller tyre height, will improve the car’s stability on corners, as well as increasing the speed such corners can be safely undertaken.
Capacity for larger brake discs. Increasing the rim diameter makes it possible to accommodate larger car brake discs and calipers with a greater number of pistons. Such additions significantly improve the effectiveness of a car’s braking system.
Improved aesthetics. Larger rims and wider tyres, as far as many drivers are concerned, readily improve the visible appearance of a vehicle.
Replacement tyres-tables
Tables of manufacturer-recommended tyre sizes and working pressures can be found near the fuel cap or on the door sills.
Disadvantages of increasing wheel diameter
Cost. New rims and new tyres always represent a significant expense, particularly since larger diameter rims are more expensive. The same applies to tyres. To reduce their height and increase the tread width, manufacturers have to use better materials and more complex tyre construction technologies. This, consequently, pushes up the price of the final product.
Increased fuel consumption. Wide tyres have a larger rolling resistance, which increases the amount of fuel consumed. Also, unless they are classed as “lightweight”, the rims will have a negative impact on performance, reducing the vehicle’s acceleration.
Higher probability of damage to the tyres and rims. When the tyre height is reduced, there is a greater likelihood of damage to the rim, or the tyre itself, when driving over potholes. Tyres with a lower profile suffer damage to their sides more often.
Reduced comfort. Reducing the tyre height makes travelling less comfortable. Even when the width of a tyre can be increased, while staying within its original permissible tolerance, the more rigid sides of a tyre with a smaller height and an increased load (doe to the tyre width) results in a significant drop in tyre comfort.
Increased danger from ruts. An increased tyre width means poorer handling when encountering ruts in the road. The more rigid sides of a low-profile tyre, which offer an advantage for precise cornering at high speed, actively impair the vehicle’s ability to drive out of ruts, this significantly reducing the safety of the car’s driver and passengers.
Increased noise levels. This depends on the shape and type of the tyre tread, but increasing the width of a tyre nearly always brings about an increase in noise. This is because the air moving between the tread blocks has an increased travel distance.
Reduced resistance to aquaplaning. A lower and wider tyre creates a larger contact area with the road surface. This increases the quantity of water that the tread needs to expel from beneath the wheel. Manufacturers try to counteract this by using directional treads or, as is becoming increasingly more common, asymmetric treads, in which two halves of the tread can be designed independently of each other, so as to fulfil different requirements. Because of this complex design, increasing the tyre width often has a significant effect on the tyre’s resistance to aquaplaning.
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.
wheel-diameter-table
To retain the same overall wheel diameter, when larger rims are fitted the tyre height must be reduced.
Changing tyre width without changing the rim size
It is possible to change the tyre width without altering the size of the rim. To do this, you just need to remember the rule for choosing replacement tyres, which says that the outer diameter of the tyre can be increased within the limits of less than 2% relative to the factory size.
More specifically, according to this rule - which, while old, can still be very much applied today - the outer diameter of a fitted tyre should not be smaller than 2%, or larger than 1.5%.
As an example, let’s assume you are looking to replace a 185/60 R15 tyre. In this situation, you could use a 195/55 R15, as this offers a diameter that is smaller than 1.3% (which is within the 2% tolerance). Similarly, you could use a 205/55 R15 tyre, for a diameter that is larger by 1% (well within the 1.5% limit).It would not be viable, however, to use the following tyre sizes:
195/50 R15 (as this is smaller by 4.25%)
165/70 R15 (as this is larger by 1.96%)
175/70 R15 (as this is larger by 4.25%)
On the other hand, you could choose more narrow tyres, such as 175/65 R15, so long as the outer diameter still respects the +1.5%/-2% range in terms of differences.
When the permissible range of replacements is calculated for size 185/60 R15 - taking the new rules from ETRTO (European Tyre and Rim Technical Organisation) from July 2003 into account - the number of possible replacements is further reduced. According to this rule, the outer diameter should not deviate more than above or below 3% when calculating the difference between the outer nominal diameters.
However, you should also remember that this rule does not mean an increased tolerance for selected replacements to 3%, as many industry websites and search tools wrongly assume.
In order to calculate the range of possible replacements for 185/60 R15 tyres. It is necessary to know both the theoretical (designed) outer diameter, as well as the nominal diameter, of the wheel rim. These need to fed into a special formula, of which a precise calculation has been provided below.
