How This Calculation Works
A tire size like 225/45R17 encodes three numbers: the width in millimeters (225), the aspect ratio as a percentage of that width (45%, meaning the sidewall height is 45% of 225 mm), and the rim diameter in inches (17). This calculator uses those three values to compute the tire's overall diameter and circumference.
The sidewall height is calculated as width × (aspect ÷ 100), converted from millimeters to inches, then doubled (since the sidewall appears above and below the rim) and added to the rim diameter to get the total tire diameter. This is then multiplied by π to get the circumference — the distance the tire covers in one full rotation.
Changing tire diameter affects your speedometer accuracy, because factory speedometers are calibrated for the original tire size's circumference. A larger tire covers more distance per rotation than the original, so the speedometer — which counts rotations — will under-read your actual speed; a smaller tire causes it to over-read.
Common Mistakes to Avoid
- Changing tire size without checking clearance. A larger diameter or wider tire can rub against the fender, suspension, or wheel well, especially when turning or over bumps. Always verify physical clearance before committing to a size change.
- Ignoring load rating changes. Different tire sizes often come with different load index ratings. Make sure any replacement size still meets or exceeds your vehicle's required load rating.
- Not accounting for speedometer and odometer error. A significant size change can throw off your speedometer and odometer readings, which may cause you to unknowingly speed, and can affect warranty mileage tracking.
- Assuming a small percentage difference is negligible. Even a 3% diameter difference is generally considered the maximum acceptable deviation by most tire professionals — larger differences can affect handling, ABS/traction control calibration, and drivetrain components on all-wheel-drive vehicles.
Worked Example
Scenario: Comparing an original 225/45R17 tire to a new 235/40R18 tire.
Step 1 — Original sidewall: 225 × 0.45 = 101.25 mm = 3.99 in. Diameter = 17 + (2 × 3.99) ≈ 24.97 in.
Step 2 — New sidewall: 235 × 0.40 = 94 mm = 3.70 in. Diameter = 18 + (2 × 3.70) ≈ 25.41 in.
Step 3 — Difference: (25.41 − 24.97) ÷ 24.97 × 100 ≈ 1.76% larger diameter.
Result: The new tire is about 1.76% larger, meaning your speedometer will read about 1.76% slower than your actual speed — a small, generally acceptable difference within the typical ±3% tolerance most professionals recommend.