Replacement tyres: the pros and cons of changing wheel sizes
- tuesday, 12 september 2011
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Find out how to choose replacement tyres properly. Check out the pros and cons of using replacements. Read a guide to changing wheel rim and tyre sizes.
What should I bear in mind when choosing replacements?
- Vehicle manufacturers permit the outer tyre diameter to be varied within a range of less than 2%. Therefore, if fitting wheel rims with larger diameter, it is necessary to reduce the profile of the tyres so as to maintain the same overall wheel diameter.
- Before making changes, remember that in most cases the use of certified tyre sizes is the only guarantee of retaining the manufacturer’s specified traction parameters.
Advantages of increasing wheel diameter
- Increased adhesion. The degree of improvement of adhesion depends to a large extent on the amount of the change in tyre profile and width. In addition, increasing the tyre width will shorten the braking distance on a dry road
- Improved reaction and behaviour on turns. An increased rim diameter combined with a smaller tyre profile will improve the stability of the vehicle’s steering on turns, and will also increase the speed at which bends can be taken safely
- Improved appearance. Larger wheel rims and wider tyres make the vehicle look better
- Possibility of fitting larger brake discs. Increasing the wheel rim diameter makes it possible to fit much larger brake discs and calipers with a larger number of pistons. These modifications significantly improve the performance of the braking system
Disadvantages of increasing wheel diameter
In order to retain the same total wheel diameter, when fitting larger rims it is necessary to reduce the profile of the tyre.
- Cost. New rims and tyres are always a significant expense, particularly since larger diameter rims are more expensive. The same applies to tyres – the reduced profile and wider tread mean that the manufacturer has to use better materials and more complex technology, which increases the cost of the final product
- Increased fuel consumption. Wider tyres produce greater rolling friction, which leads to an increase in the amount of fuel consumed. In addition, if the wheel rims are not of lightweight type, these will also have a negative impact on performance (reducing the vehicle’s acceleration)
- Increased likelihood of tyre and wheel damage. The reduced tyre height means a greater chance of damage to the tyre itself or to the wheel rim if the vehicle hits a pothole. Low-profile tyres are often at greater risk of damage to their sides
- Reduced comfort. Reduction in the tyre profile reduces travel comfort. Even when it is possible to widen tyres while maintaining their height within the original tolerance limits, the more rigid sides of low-profile tyres and the increased load due to the width of the tyre will cause the comfort of driving on such tyres to be significantly reduced
- Increased danger from ruts. Wider tyres make it more difficult to manoeuvre in case of ruts in the road. Also the more rigid sides of a low-profile tyre, which are an advantage when making precision turns at high speed, reduce the vehicle’s ability to drive out of ruts, thus significantly impacting on the safety of driver and passengers
- Increased noise. This depends on the shape and type of tread, but wider tyres are almost always noisier, because the air passing between the lugs of the tread has a longer distance to travel
- Increased susceptibility to aquaplaning. A lower, wider tyre means a larger contact surface, and hence a larger quantity of water which the tread needs to “pump out” from beneath the wheel. Manufacturers try to counteract this by using directional tread patterns or, increasingly often, asymmetric ones, enabling the two halves of the tread to be designed independently of each other, taking account of their different requirements. Increased tyre width has a significant impact on susceptibility to possible aquaplaning
- Loss of adhesion on a dirty road. Increased tyre width is almost always accompanied by an increase in braking distance on a sandy or dirty road
Changing tyre width without changing rim size
Changing the width of your tyres does not necessarily entail changing the size of the wheel rims. Remember, though, to keep to the rule that applies when choosing replacements, namely that the outer diameter of the tyre can be increased within limits of less than 2% relative to the factory size.
According to the old rule for choosing replacements (which can indeed still be applied), the outer diameter of a fitted tyre should not be smaller by more than 2% or larger by more than 1.5%. For example, in place of a 185/60 R15 tyre, you may use a 195/55 (diameter smaller by about 1.3%) or a 205/55 (diameter larger by about 1%). However it would be inappropriate to use, for example, a 195/50 R15 (diameter 4.25% smaller), 165/70 R15 (diameter 1.96% larger), or 175/70 R15 (diameter 4.25% larger). It is however permissible to use narrower tyres, such as a 175/65 R15, while staying within the +1.5/–2% tolerance limits for the difference in outer tyre diameter.
When the range of possible replacements for 185/60 R15 tyres is calculated according to the new rules as recommended by ETRTO since July 2003, the number of possibilities is reduced. According to these rules the outer tyre diameter should deviate by within +/–3% of the difference between the theoretical (designed) outer diameter and the nominal diameter of the wheel ring. It should be remembered that this does not imply an increase in the tolerance for selection of replacements to 3%, as is often wrongly stated by the search tools for replacements provided on some industry websites.
In order to calculate the range of possible replacements for 185/60 R15 tyres, it is necessary to know such data as the theoretical (designed) outer diameter and the nominal diameter of the wheel ring, and to feed these into a special formula. A precise calculation is given below.
- Theoretical/designed outer diameter: 603 mm
- Nominal diameter of wheel ring: 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 keeping the outer diameter within the limits +1.5/-2%, it turns out that the outer diameter of a replacement tyre can lie 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 that the number of possible replacement tyres is reduced. The available sizes no longer include 195/55 R15.
When taking these decisions, consider the effects of using certified replacement tyres on your vehicle. By using wider tyres with a lower profile, although you gain in terms of adhesion and better steering (the side wall of the tyre is lower and more rigid), these changes also lead to an increased likelihood of aquaplaning at low speeds, increased rolling friction leading to higher fuel consumption, decreased travelling comfort, and poorer on-the-spot manoeuvrability. You must also remember to ensure that the wider tyres do not rub against the car wing when the wheels are at full turn.
The only way to ensure optimum vehicle safety is to fit suitable tyres that have been certified by the manufacturer. Fitting narrower tyres in turn may lead to reduced adhesion, increased braking distances, and less stability on turns due to the larger tyre profile and incorrect operation of the ABS system, particularly with winter tyres.
For this reason the most sensible solution is either to retain the same tyre diameter, which obviously does not offer a great choice of sizes, or to fit tyres with a diameter larger by one inch.
Reducing wheel rim size
This is usually done when winter tyres are used. The same principles apply as when increasing the rim diameter, although you must also check whether the sizes of the brake discs and calipers are such as to permit the fitting of smaller rims.
The operation makes it possible to fit tyres that are narrower and have a higher profile, which is sometimes a more effective solution for winter tyres. You will feel the greatest benefit from this in your wallet – tyres with smaller diameter are normally cheaper. It is sometimes claimed that reducing the size of winter tyres increases their adhesion in winter conditions, due to the increased unit pressure per square centimetre. It is not recommended to reduce the size of winter tyres to a tread width of 205 mm. The benefits of doing so will be negligible, and it can often reduce a car’s adhesion and negatively affect the operation of systems such as ABS. In the case of treads wider than 215 mm you can consider taking such a step, but remember that all of the vehicle’s systems will function optimally only if the vehicle’s original, manufacturer-certified tyre types are retained.