In this article, we will explore how to properly choose replacement car tires. When it comes to changing wheel and tire sizes, it helps to understand all the possible advantages and disadvantages.
Replacement Car tires refer to alternative tire sizes that can be used safely on your vehicle. These replacements are used for a number of reasons, notably:
- To alter certain driving parameters, such as braking distances.
- To improve the vehicle’s appearance, via large rim diameters.
- To make savings, as smaller tires offer a better fuel economy.
Contents 1. What to bear in mind when choosing replacement tires 2. Advantages of increasing wheel diameter 3. Disadvantages of increasing wheel diameter 4. Changing tire width without changing the rim size 5. Reducing rim sizes
What to Bear in Mind When Choosing Replacement Car Tires
Vehicle manufacturers do permit the outer tire 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 tires to compensate and maintain the overall combined diameter.
Before making any changes, remember that, in most cases, the use of officially approved tire 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.
Increased grip. The degree of grip improvement depends, to a large extent, on the amount of change in the tire height and width. Also, increasing the width further shortens the braking distance on dry roads.
Improved reaction and behavior on turns. An increased rim diameter, combined with a smaller tire 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 tires, as far as many drivers are concerned, readily improve the visible appearance of a vehicle.
Tables of manufacturer-recommended tire 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 tires always represent a significant expense, particularly since larger-diameter rims are more expensive. The same applies to tires. To reduce their height and increase the tread width, manufacturers have to use better materials and more complex tire construction technologies. This, consequently, pushes up the price of the final product.
Increased fuel consumption. Wide tires 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 tires and rims. When the tire height is reduced, there is a greater likelihood of damage to the rim, or the tire itself, when driving over potholes. Tires with a lower profile suffer damage to their sides more often.
Reduced comfort. Reducing the tire height makes traveling less comfortable. Even when the width of a tire can be increased, while staying within its original permissible tolerance, the more rigid sides of a tire with a smaller height and an increased load (doe to the tire width) results in a significant drop in tire comfort.
Increased danger from ruts. An increased tire width means poorer handling when encountering ruts on the road. The more rigid sides of a low-profile tire, which offer an advantage for precise cornering at high speed, actively impair the vehicle’s ability to drive out of ruts, this significantly reduces the safety of the car’s driver and passengers.
Increased noise levels. This depends on the shape and type of the tire tread, but increasing the width of a tire 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 tire 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 it is becoming increasingly more common, asymmetric treads, in which two halves of the tread can be designed independently of each other, to fulfill different requirements. Because of this complex design, increasing the tire width often has a significant effect on the tire’s resistance to aquaplaning.
The reduced grip on dirt roads. When the tire width is increased, there is nearly always an increase in the braking distance on sandy or dirty roads.
To retain the same overall wheel diameter, when larger rims are fitted the tire height must be reduced.
Changing Tire Width Without Changing the Rim Size
It is possible to change the tire width without altering the size of the rim. To do this, you just need to remember the rule for choosing replacement tires, which says that the outer diameter of the tire 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 tire 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 tire. 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 tire, for a diameter that is larger by 1% (well within the 1.5% limit). It would not be viable, however, to use the following tire 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 tires, 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 ET RTO (European Tyre and Rim Technical Organization) 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 tires. It is necessary to know both the theoretical (designed) outer diameter, and the nominal diameter, of the wheel rim. These need to feed into a special formula, of which a precise calculation has been provided below.
Theoretical (designed) outer diameter 603 mm
Nominal diameter of rim 381 mm
Difference: 222 mm
Maximum permissible diameter: 603 + (222 x 3%) = 609,66
Minimum permissible diameter: 603 – (222 x 3%) = 596,34
When the older method for calculating possible replacements is applied, based simply on limitations of +1.5/-2%, it turns out the outer diameter of a replacement tire (still using the example of 185/60 R15) lies between 598.78 mm and 620.165 mm. When the new +3/-3% formula is applied, however, the range is narrowed to values between 596.34 mm and 609.66 mm.
This means a number of possible replacement tires are no longer viable. The available sizes, for example, no longer include 195/55 R15.
When undertaking these decisions, consider the effects of using officially certified replacement tires on your vehicle. By using wider tires with a lower profile, despite any gains in terms of grip and improved steering, the changes also lead to an increased likelihood of aquaplaning at low speeds, increased rolling resistance, less driving comfort, and poorer on-the-spot maneuverability. You must also remember to ensure that the wider tires do not rub against the car when the wheels are at their full turning position.
Ultimately, the only way to ensure optimum vehicle safety is to fit suitable tires that have been certified by the manufacturer. Fitting narrower tires, in turn, can lead to reduced grip, increased braking distances, and less stability on turns due to a larger tire profile and an incorrectly operating ABS system. This is particularly important with winter tires when such criteria are highly important.
For this reason, the most sensible solution is to either remain in the same tire diameter, which naturally limits your choice of sizes, or to fit tires with a diameter larger by one inch.
Calculating possible replacements for size 185/60R15 tires:
- Max. Diameter -2% +1.5% Theoretical diameter -3% +3%
- 185/60 R15 84 T/H 88 T/H 611 mm 598.78 mm 620,165 mm 603 mm 596.34 mm 609.66 mm
- 195/55 R15 85 T/H/V 89 603 mm OK 595 mm Outer diameter too small
- 205/55 R15 88 V/W 617 mm OK 607 mm OK
- 175/65 R15 84 T/H 619 mm OK 609 mm OK
- 195/50 R15 82 T/H 86 V 585 mm Outer diameter too small 577 mm Outer diameter too small
- 165/70 R15 82 623 mm Outer diameter too large and load index too small 613 mm Outer diameter too large and load index too small
- 175/70 R15 86 637 mm Outer diameter too big 627 mm Outer diameter too big
Reducing Rim Sizes
This is usually done in the colder season with winter tires, yet the same principles apply when reducing the rim size. In addition, you must also check whether the size of your brake discs and calipers are able to permit the fitting of smaller rims.
It is possible to fit tires that are narrower and have a higher profile, which is sometimes a more effective solution for winter tires. You feel the greatest benefit through your wallet, as tires with a smaller diameter are normally cheaper.
It is sometimes claimed that reducing the size of winter tires increases their grip in winter conditions, due to the increased tire pressure, in terms of units per square centimeter. However, it is not recommended to reduce your tire size to a read width of 205 mm or less. The benefits of doing so will be negligible, and it can often reduce a car’s grip and negatively affect the operation of various systems, such as ABS.
In the case of treads wider than 215 mm, you can consider taking such a step. Just remember that all the vehicle’s systems will only function optimally if the vehicle’s original, manufacturer-certified tire types are retained.