Tyre Size Calculator

A tyre size calculator decodes the alphanumeric tyre size markings stamped on a tyre's sidewall and calculates key dimensions including section width, aspect height, overall diameter, and circumference. It is used by motorists, mechanics, and automotive enthusiasts to compare tyre sizes, check speedometer accuracy, and find compatible alternatives when changing wheel sizes.

How to Use the Tyre Size Calculator

1. Find the tyre size on the sidewall of your existing tyre. It will appear in a format such as 205/55R16 91V.
2. Enter the three main numbers: section width, aspect ratio, and rim diameter.
3. Click "Calculate" to see the tyre's dimensions: section width in mm, sidewall height, overall diameter, and circumference.
4. To compare two tyre sizes, enter the second size in the comparison fields and see the percentage differences in diameter and circumference.
5. Check the speedometer error percentage to understand how a size change affects your displayed speed.

The Formula

Standard tyre notation: [Width]/[Aspect Ratio]R[Rim Diameter]

Example: 205/55R16

Where:

• 205 = section width in millimetres (the width of the tyre from sidewall to sidewall)
• 55 = aspect ratio (the sidewall height as a percentage of the section width)
• R = radial construction (the standard for modern passenger tyres)
• 16 = rim diameter in inches

Calculations:

Sidewall Height (mm) = Section Width x (Aspect Ratio / 100) = 205 x (55 / 100) = 205 x 0.55 = 112.75 mm

Overall Tyre Diameter (mm) = (Sidewall Height x 2) + (Rim Diameter x 25.4) = (112.75 x 2) + (16 x 25.4) = 225.5 + 406.4 = 631.9 mm

Tyre Circumference (mm) = Pi x Diameter = 3.14159 x 631.9 = 1,985 mm (approximately 1.985 metres per revolution)

Speedometer Error (%) = ((New Circumference - Original Circumference) / Original Circumference) x 100

Real-World Example

Your car currently runs 205/55R16 tyres and you want to fit 225/45R17 alloys. Will these affect your speedometer?

Original tyre (205/55R16):

• Sidewall: 205 x 0.55 = 112.75 mm
• Diameter: (112.75 x 2) + (16 x 25.4) = 225.5 + 406.4 = 631.9 mm

New tyre (225/45R17):

• Sidewall: 225 x 0.45 = 101.25 mm
• Diameter: (101.25 x 2) + (17 x 25.4) = 202.5 + 431.8 = 634.3 mm

Difference: 634.3 - 631.9 = 2.4 mm (0.38% larger)

This is well within the acceptable tolerance of plus or minus 3%, so your speedometer will read almost identically. Your actual speed at an indicated 100 km/h will be approximately 100.4 km/h.

Understanding Tyre Markings

After the three main numbers, the sidewall includes additional codes. The load index (e.g., 91) indicates the maximum load the tyre can carry per wheel; 91 corresponds to 615 kg. The speed rating (e.g., V) indicates the maximum sustained speed; V means 240 km/h. Letters after the speed rating include: Q (160 km/h), S (180 km/h), T (190 km/h), H (210 km/h), V (240 km/h), W (270 km/h), Y (300 km/h). Some tyres are marked XL (extra load) or RF (run-flat). Never fit a tyre with a lower load index or speed rating than specified in your vehicle handbook. Fitting the wrong tyre category can void your insurance and is illegal in many jurisdictions.

Frequently Asked Questions

How much can I deviate from my original tyre size? Most tyre fitting professionals and vehicle manufacturers recommend keeping the overall diameter within 3% of the original size. Going outside this range affects speedometer accuracy, ABS calibration, and ground clearance. Always consult your vehicle's type approval documentation or the inside of the driver's door frame for the permitted tyre size range.

What is the purpose of the aspect ratio? The aspect ratio determines the sidewall height. A low aspect ratio (e.g., 35 or 40) means a shorter, stiffer sidewall, which improves handling precision and cornering feedback but gives a harsher ride and less cushioning over potholes. A higher aspect ratio (e.g., 65 or 70) provides a taller, more compliant sidewall with better comfort and ride quality, typically found on family and economy cars.

Does changing tyre size affect fuel economy? Yes. A larger diameter tyre has more rotational inertia, requiring slightly more energy to accelerate. Wider tyres have more rolling resistance. However, the effect is small for modest size changes. Tyre compound and inflation pressure have a greater influence on rolling resistance and fuel economy than small size variations.

Can I mix tyre sizes on the same axle? No. Tyres on the same axle must always be the same size, brand, and ideally the same compound. Mixing sizes causes uneven traction, instability, and differential stress that can damage drivetrain components. On all-wheel-drive vehicles, all four tyres must match within a tight tolerance or severe mechanical damage to the centre differential can result.

Keeping the diameter inside the tolerance

The size comparison in the tool answers a specific question: if I fit this tyre instead, how far has the rolling diameter moved? The answers below all use 205/55R16 as the original, with a sidewall of 112.75 mm and an unloaded diameter of 631.90 mm.

Eight of the thirteen sizes sit inside the plus or minus 3% band that fitters and manufacturers work to. Five do not, and four of those are more than 3.5% out. A size whose diameter grows to 652.40 mm has added 20.50 mm, which is a different class of change from the 2.40 mm that the 225/45R17 brings.

What a size change does over distance

Two figures make the effect concrete.

The speedometer error is a percentage, so it scales with speed. A tyre 0.38% larger than the original reads 0.38 km/h low at an indicated 100 km/h, and 0.76 km/h low at an indicated 200 km/h. That is inside the tolerance a speedometer is allowed by construction, which is why a small change is often invisible in daily driving.

The odometer error is the same percentage applied to distance. Over 10,000 km, a 0.38% change adds 38 km to the distance the car reports. Revs per kilometre move in the other direction: this tyre turns 501.83 times per kilometre against the original's 503.73, which is 1.91 fewer turns over the same road.

Reading the load index and speed rating

The load index and the speed rating sit after the size marking on the sidewall, as in 205/55R16 91V. The three numbers go in the size fields. The other two come off the tyre and are checked against the vehicle handbook separately.

The speed rating is a letter, and the common ones for passenger cars run from Q at 160 km/h through S at 180, T at 190, H at 210, V at 240, W at 270 and Y at 300. Fitting a lower index or a lower rating than the handbook specifies reduces the load or speed the vehicle is approved for, and on many vehicles it makes the fitment illegal as well as unsafe.

Why the static diameter is an approximation

The tool multiplies the rim diameter in inches by 25.4 and adds two sidewall heights. That gives the unloaded diameter, which is the figure stamped on specification sheets and the right figure for judging whether a fitment is close.

A tyre in service deflects under load, so its rolling radius sits below the unloaded radius. The result is that the distance covered in one revolution is less than pi times the unloaded diameter, and tyre makers publish revolutions per kilometre measured under load rather than calculated from the sidewall. Two consequences follow for anyone reading the output.

• A diameter comparison tells you whether a size is close enough to fit. It does not tell you the exact speedometer error, because the true rolling circumference depends on load, inflation pressure and construction.
• The speedometer check is a good approximation and not a calibration. If the error matters, the number to use is the tyre maker's published revolutions per kilometre for both tyres.

Two further rules sit outside the arithmetic. Tyres on the same axle must match in size, and on an all-wheel-drive vehicle all four tyres need to be within a tight diameter tolerance or the centre differential takes the strain. Check the permitted size range on the vehicle's type approval document or the label inside the driver's door frame before ordering.

The designation system used above, including the meaning of the section width, aspect ratio and rim diameter and the load capacity table, is defined by the European Tyre and Rim Technical Organisation in its Standards Manual. The load and speed limits are the manufacturer's published values for the sizes listed.

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