Tyre Size Calculator: Compare Tyre Sizes Online

Planning to fit tyres of a different size? The tyre size calculator on skad.in.ua helps you compare your current and proposed tyre sizes, assess changes in overall wheel dimensions, and understand what to consider before buying.

The tool is useful when choosing tyres for a passenger car, SUV, or commercial vehicle. It can help you assess an alternative size if the tyres you need are unavailable or you are considering switching to wheels of a different diameter.

Your vehicle manufacturer’s requirements should always be the starting point. The calculator compares tyre geometry, but it does not confirm that a chosen size is suitable for a particular vehicle. Before fitting, check the approved tyre sizes, wheel specifications, and tyre load and speed ratings.

How to Use the Tyre Size Calculator

You will need two tyre sizes to make a comparison: the size currently fitted to your vehicle and the size you plan to use. You can find the size marking on the tyre sidewall. For example, in 205/55 R16:

• 205 is the nominal tyre width in millimetres;
• 55 is the sidewall height as a percentage of the tyre width;
• R indicates radial tyre construction;
• 16 is the rim diameter in inches.

Enter the original size, then specify the dimensions of the new option. The comparison will help you assess changes in tyre width, sidewall height, and overall wheel diameter.

Please note: the same aspect ratio does not mean the same sidewall height. Sidewall height depends on tyre width, so when changing one parameter, it is important to consider the others.

What to Consider When Comparing Tyre Sizes

When choosing an alternative tyre size, checking the rim diameter alone is not enough. The new tyre must be compatible with the wheel and meet the vehicle’s requirements, and the complete wheel and tyre assembly must fit within the wheel arch with adequate clearance.

Overall Wheel Diameter

A change in overall diameter affects the distance the vehicle travels with each wheel revolution. This can affect speedometer and odometer readings. If the diameter increases, the actual speed at the same speedometer reading will be higher than with the original size; if the diameter decreases, it will be lower.

Ground clearance may also change. All other factors being equal, the change in ground clearance is approximately half the difference between the overall wheel diameters.

Tyre Width

Wider tyres require checks for wheel compatibility and sufficient space within the wheel arch. Make sure the wheel and tyre assembly does not touch suspension or body components when steering or as the suspension moves.

Increasing tyre width alone does not guarantee better grip. Vehicle behaviour also depends on tyre construction, rubber compound, tread pattern, tyre pressure, and road conditions.

Sidewall Height

Sidewall height affects how the wheel absorbs road irregularities. Switching to a lower-profile tyre may make potholes and road joints feel more pronounced and increase the risk of tyre or wheel damage during a severe impact.

A higher-profile tyre changes the vehicle’s response and the overall wheel dimensions. It should therefore be chosen from the sizes specified by the manufacturer.

How to Choose Tyres When Changing Wheels

If you want to fit larger-diameter wheels, you will need to recalculate the tyre size. Maintaining an overall wheel diameter close to the original specification usually requires reducing the sidewall height.

However, matching the overall diameter is only part of the check. When choosing wheels, you need to consider:

• rim diameter and width;
• the number of bolt holes and their pitch circle diameter — PCD;
• wheel offset — ET;
• centre bore diameter — DIA;
• permitted load and compatibility with the braking system.

Increasing wheel diameter does not mean the wheel offset must necessarily be reduced. Offset is selected separately, taking into account the vehicle’s requirements and the wheel design.

If the wheel’s centre bore is larger than the hub diameter, whether centring rings can be used depends on the wheel type and mounting method. A wheel with a smaller centre bore cannot be fitted directly. Do not modify the wheel’s centre bore or the hub dimensions yourself.

Converting Tyre Sizes from Inches to Metric

Some off-road and other tyres use inch-based size markings. For example, in 31×10.5 R15, 31 indicates the nominal overall tyre diameter in inches, 10.5 is the nominal tyre width, and 15 is the rim diameter.

Converting the units helps you compare this size with familiar metric tyre markings. However, the calculated values should be treated as a guide: sizes that appear similar in calculations may differ in their actual dimensions and fitting requirements.

What to Check Before Buying Tyres

The tyre size calculator is useful for preliminary comparisons, but a calculated size may not be commercially available or approved for your vehicle.

Before ordering, check:

• whether the chosen tyre size is approved for the vehicle;
• whether the tyre is compatible with the wheel’s width and diameter;
• whether the load and speed ratings are suitable;
• whether there is sufficient clearance from the body, suspension, and brake components;
• whether there are specific requirements for different axles or the all-wheel-drive system.

For commercial vehicles, also consider axle loads, operating conditions, and requirements for single or dual wheel fitment.

Use the tyre size calculator on skad.in.ua to compare sizes before buying. Make your final choice based on the vehicle documentation and the specifications of the particular tyres and wheels to help avoid fitting errors.