Watch out for your speedometer
If you fit a tyre equivalent with a bigger diameter, this can cause the speedometer to misrepresent your actual velocity. This change depends on the change in tyre size. If you fit tyres with a larger diameter, the speedometer will show a figure lower than your actual speed. If you fit tyres with a smaller diameter, the speedometer will instead show a higher than realistic speed.
Alternatives vs Indexes
Our tyre size converter helps identify alternative sizes by using restricting diameter sizes to the specified 1.5 or -2% rules mentioned earlier. However, this will just show you all the theoretically possible sizes.
Aside from the acceptable tyre diameter difference, you should also consider the various indexes and factors that represent a tyre’s suitability. This is important when choosing new car tyres in your chosen size and ensuring they are within the accepted index limits.
Tyre alternatives dictionary
Not sure what some of the numbers on your tyre mean, or how to identify the various indexes? We’ve prepared a small dictionary of these terms for your benefit.
Tyre Size - The tyre size is easy to read, once you know how. Simply put, it is made up of a series of numbers. Let’s use 205/55 R16 as an example. The first value (in this example, 205) is the tyre width in millimeters. The second value (55) is the tyre profile in percentage. The third value (16) is the rim diameter in inches. The letter at the start of the third value refers to the type of tyre construction. R, for example, refers to radial tyres.
Speed Index - Known as SI for short, the speed index determines the maximum speed allowed for a given tyre. The tyre speed index is expressed with letters, each having a specific limit, such as:
● T (118 mph)
● H (130 mph)
● V (140 mph)
It should be noted that, although T, H and V are all above 70 mph - the national driving limit in the UK - you still require an index that matches the car manufacturer’s specifications. This often means having tyres suitable for your car’s top speed, even if you are never planning to drive that fast.
This is clearly marked on the side your tyres. Just look up the assigned letter on our tyre speed index.
Load Index - Referred to as LI for short, this figure determines the maximum load that the tyre can handle when driving at the maximum speed (as determined by the aforementioned speed index).
This is expressed as a number, which can be checked up on our tyre load rating index. Just like the speed index, this information can be found on every tyre - it is found on the tyre sidewall.
Outside tyre diameter - This figure is the total tyre height, measured from the ground to the top of the tread. This is slightly different from the internal diameter, as this is the complete external diameter of the tyre and wheel.
The tyre size calculator does not include information about car make and model. Please keep in mind your car's capabilities..
When it comes to choosing a new size, many drivers choose to stay within the same size and diameter - these tyres are known as equivalents. Alternatives, on the other hand, have a different size. They are typically larger - upgrading in this way is referred to as plus sizing.
What tyre size is acceptable?
When using a tyre size comparison tool, it’s important to know the difference in the outer diameter. Our tool gives this in percentages: a positive number is larger/wider, while a minus number represents a negative change.
When it comes to these results, it is recommended that you only use options within 1.5% and -2%.
When should you change tyre size?
Not everyone needs to change tyre size. For most drivers, this is done to achieve or improve certain driving parameters on their vehicle. An alternative sizes can offer various improvements, such as a shorter braking distance, as well as drawbacks, so it is important to understand what it is you want from your car tyres.
Find the correct tyre sizes for your vehicle.
What else do you need to consider?
Our tyre diameter calculator is a great tool for finding a new tyre size, but you should keep in mind some physical limitations. Our tyre calculator does not, for instance, take into account your specific car or model. As such, always keep the physical needs and parameters of your vehicle in mind.
Likewise, you need to be careful when changing the diameter of the tyre. You can’t replace a standard 225/40 R18 tyre, for instance, with a 185/80 R13 tyre, as the smaller model would require 13 inch wheels that can’t fit the braking discs from the larger 18 inch wheels.
Similarly, you couldn’t go up from a 185/80 R13 to a 225/40 R18 tyre due to a look of space in the wheel arch. Cars just aren’t designed to accommodate such great changes, which is why rules such as the 1.5% or -2% are regularly used.
Your safest solution is to choose equivalent options with the same rim diameter. However, if you do wish to use a different rim size, our specialists are always available to help.
The standard size
This tyre size converter is designed for comparing new tyre sizes against the original size - the option recommended by the car’s manufacturer, as stated in its manual.
Because of this, it is vital that you know the original tyre size for your vehicle. If you already have alternative tyre sizes, you can’t use these for the calculator, as you will quickly stray out of the safe limits for size difference, among other issues. For example, while it might be within 1.5% of the previous replacement, it may very well be outside this range compared to the original tyre and, consequently, the car’s intended design.
tyre size configurating tool.
Pros and cons of wide tyres
- - Improved grip
- - Shorter braking distances
- - Faster car response times and improved cornering stability
- - Higher tyre price
- - Higher fuel consumption
- - Increased vibrations
- - More tyre noise
- - Lower water draining and decreased aquaplaning resistance
Pros and cons of narrow tyres
- - Higher resistance to aquaplaning
- - Increased driving comfort and less tyre noise
- - Lower tyre price
- - Better driving in winter conditions (especially if you’re using a winter tyre)
- - Reduced grip
- - Worse behaviour on corners
- - Longer braking distance
Virtually any car can benefit from our tyre size convertor. Although manufacturers produce cars with a particularly recommended tyre size, you can always search for optional alternatives. As mentioned earlier, this can produce different benefits and drawbacks, helping you to customise your driving experience via your vehicle’s performance parameters.
Choosing an alternative tyre size usually requires changing both the tyre and wheel diameter. If you fit a lower tyre diameter, the overall profile will automatically be lower than your previous tyres, unless you use a wider wheel to compensate (this, of course, would require a tyre with a matching internal diameter).
You should also always be aware that, after changing tyre size, various driving characteristics, such as the various fuel consumption and grip properties, well change as a result. These may be better or worse, depending on the options you have chosen.
As we have already mentioned, there are pros and cons to consider, whether you choose a wider or narrower tyre.
Likewise, it is advised you stick within the 1.5% and -2% change in size. Incorrect tyre sizing can cause damage to the car. Bigger rims, or wider tyres, will make contact with the wheel arch, causing excess wear and tear, as well as more difficult driving. This can even damage further car parts, such as the suspension system.
For these reasons, it’s always best to use our tyre size comparison tool to ensure the change is not too drastic.