Tire Size Calculator
Compare tire sizes, calculate diameter, and check speedometer difference.
This tire size calculator reads a standard tire size code (like 225/45R17) and converts it into overall diameter, sidewall height, and circumference, so you can check fitment or compare it against another size. It doubles as a tire size conversion calculator for comparing an old tire size to a replacement.
Enter a tire size code, or two codes to compare, and the calculator returns the diameter difference along with the resulting speedometer error percentage.
Worked Calculation Examples
| Scenario | Result | Calculation Step |
|---|---|---|
| 225/45R17 | ≈24.97 in diameter | Sidewall height = 225 × 0.45 = 101.25 mm ÷ 25.4 ≈ 3.99 in. Diameter = 17 + (2 × 3.99) ≈ 24.97 in. |
| 205/55R16 | ≈24.88 in diameter | Sidewall height = 205 × 0.55 = 112.75 mm ÷ 25.4 ≈ 4.44 in. Diameter = 16 + (2 × 4.44) ≈ 24.88 in - very close to the first example despite a different size code, which is why these two are common alternate fitments. |
Reading a Tire Size Chart Code
A tire size code like 225/45R17 breaks down as: 225 is the tire width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 17 is the wheel diameter in inches. This tire dimensions calculator uses those three numbers to compute the tire's total diameter and circumference.
Tire Size Comparison and Speedometer Accuracy
Changing to a tire with a different overall diameter changes how far the vehicle travels per wheel rotation, which throws off the speedometer and odometer reading proportionally. A tire size comparison that shows more than roughly a 3% diameter difference is generally considered outside the safe fitment range recommended by tire manufacturers.
How to Use the Tire Size Calculator
- Open the Tire Size Calculator and enter the values requested in the input fields.
- Check the units, percentages, dates, or time periods before reading the answer.
- Review the instant result and adjust any value to compare another scenario.
- Use the formula, example, and FAQs below to understand how the tire size calculator works.
Frequently Asked Questions
How do I read a tire size?
The numbers represent width in millimeters, aspect ratio as a percentage, construction type (R for radial), and wheel diameter in inches, in that order - for example, 225/45R17.
How does tire size affect my speedometer?
A larger overall tire diameter means fewer wheel rotations per mile, causing your speedometer to under-read your actual speed; a smaller diameter causes it to over-read.
What is a safe tire size conversion range?
Most manufacturers recommend keeping the overall diameter change within about 3% of the original tire size to avoid speedometer, clearance, and handling issues.
How do I use this tire size calculator?
Enter the known values, review the units or settings, and the calculator updates the result instantly. The formula and example on this page show how the answer is produced.
What does the Tire Size Calculator calculate?
Compare tire sizes, calculate diameter, and check speedometer difference. It is designed for fast browser-based calculations without sign-up, downloads, or manual spreadsheet setup.
What formula does this tire size use?
The formula is: Diameter = Rim Diameter + 2 × (Width × Aspect Ratio ÷ 100) ÷ 25.4. This metric tire sizing code (like 225/45R17) was standardized internationally through ISO and industry cooperation in the 1970s, replacing older, less descriptive tire size conventions - the 'R' specifically denotes radial construction, which became the dominant tire technology after Michelin's 1946 radial tire patent proved superior in durability and handling to the older bias-ply designs it eventually replaced almost entirely.
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Last updated: September 27, 2026.