Tire size calculator
Enter the current and new tire sizes to compare the outside diameter, circumference and speed adjusted for the new size. Supports metric and inch formats, and shows estimates assuming the speedometer is accurate with the current tires. It does not determine whether the tires fit or meet vehicle inspection requirements.
What you can do
Supports 2 notations
Choose metric notation, such as 205/55 R16, or inch notation, such as 31 × 10.5 R16, for each tire.
Compare the same measurements side by side
Compare width, outer diameter, circumference, sidewall height, and revolutions per 1km for the current and replacement tires.
Convert speed using the outer diameter ratio
Multiply the speedometer reading by the outer diameter ratio to display the calculated speed after the size change. You can copy all results or save them as text.
How to use
Enter current and replacement sizes
Choose the notation for each tire and enter the width, aspect ratio, and wheel diameter, or outer diameter, width, and wheel diameter marked on the tire.
Change comparison settings as needed
In advanced settings, change the speedometer reading and unit and the unit for result dimensions. The defaults are 100 km/h and mm.
Compare and check the differences
Click “Compare” and check the outer diameter difference, converted speed, and each dimension. Compare again after changing the inputs.
Copy or save the results
If needed, copy all 14 comparison results together or save them as a text file. Input notations and diagrams are not included in the saved content.
Useful when
Compare dimensions of candidate sizes
When you want to compare candidate tires with different wheel diameters and aspect ratios using the same measurements, such as outer diameter and sidewall height.
Compare inch notations
When you want to compare tires marked with outer diameter and width in inches with metric tires using the same unit of length.
How to read tire notations and calculated values
How to read 205/55 R16
205 means a width of 205mm, 55 means a sidewall height of 55% of the width, and 16 means a wheel diameter of 16 inches. R denotes radial construction.
How to read 31 × 10.5 R16
Inch notation lists the outer diameter of 31 inches, width of 10.5 inches, and wheel diameter of 16 inches, in that order. You can enter them in this order on the screen. The outer diameter must exceed the wheel diameter.
Formulas for outer diameter and circumference
In metric notation, outer diameter is wheel diameter × 25.4 + width × aspect ratio ÷ 100 × 2 (mm). Circumference is outer diameter × π. In inch notation, sidewall height is half the difference between the outer diameter and wheel diameter.
Difference signs and converted speed
Differences are “replacement − current.” The percentage change in outer diameter is calculated relative to the current outer diameter. Converted speed is entered speed × replacement outer diameter ÷ current outer diameter, assuming the same wheel rotation rate.
Differences from actual vehicle dimensions and compatibility
Calculations are based on size notations. They do not account for actual dimension differences due to brand, tire pressure, load, or wear, or for the speedometer’s own error. Outer diameter difference alone cannot determine fitment or compliance with vehicle inspection requirements.
Frequently asked questions
Can I fit the tires if the outer diameter difference is small?
This calculation alone cannot determine that. Check the specified size for your vehicle, load capacity, wheel compatibility, clearance from the vehicle body, and other requirements with the retailer or manufacturer.
Is the converted speed my actual driving speed?
No. This value is calculated from the outer diameter ratio, assuming the speedometer is accurate with the current tires. It is neither a measured value nor a feature for measuring speedometer error.
Can I compare metric and inch notations together?
Yes. You can choose different notations for the current and replacement tires. Switching notation does not automatically convert values, so check the displayed values. The inputs for each notation are retained, and the wheel diameter is shared.
Are the entered values converted when I switch units?
Switching between km/h and mph keeps the numerical speed value and changes only the unit. The result units, mm and in, are converted when you compare again. Revolutions are always shown per 1km.
Will the ride height change be the same on the actual vehicle?
This calculated value is half the outer diameter difference. It does not account for load or tire deformation, so it does not guarantee the actual change in ride height or ground clearance.
Input and results
- Metric notation
- Width 50–1000mm, aspect ratio 10–150%, and wheel diameter 4–50in. Decimals are also accepted.
- Inch notation
- Outer diameter 10–100in, width 2–40in, and wheel diameter 4–50in. The outer diameter must exceed the wheel diameter.
- Speed and display
- Speed is 0–1000 km/h or mph. Dimensions are displayed in mm or in. Results are approximate values with up to 10 significant digits.
- Get results
- Copy all 14 items or save them as tire-size-calculator.txt. The tool does not determine whether tires can be fitted to the actual vehicle.