Tire Size Calculator
Compare two tire sizes' overall diameter, percentage difference, and the resulting speedometer error when swapping.
Original Tire (205/55R16)
24.88 in diameter
Sidewall height: 4.44 in
Circumference: 78.16 in
New Tire (215/45R17)
24.62 in diameter
Sidewall height: 3.81 in
Circumference: 77.34 in
Overall Diameter Difference
-1.04%
Speedometer error when swapping from the original to the new size: -1.04%. Your actual speed will be lower than what the speedometer shows (it will over-read).
How to Compare Tire Sizes
A tire size like 205/55R16 encodes its width in millimeters (205), its aspect ratio as a percentage of that width (55%, giving the sidewall height), and the rim diameter in inches (16). This calculator parses two tire sizes, computes each one's overall diameter, and compares them — including the speedometer error you'd introduce by swapping from one size to the other.
Overall Diameter (in) = Rim Diameter (in) + 2 × Sidewall Height (in)
Example
A 205/55R16 tire has a sidewall height of 205 × 0.55 ÷ 25.4 ≈ 4.44 in, giving an overall diameter of 16 + 2 × 4.44 ≈ 24.88 in. A 215/45R17 tire works out to about 24.98 in — a difference of roughly 0.4%, small enough to barely register on the speedometer.
Common Use Cases
- Checking whether a different tire size will fit without throwing off your speedometer or odometer.
- Comparing a wider/lower-profile "plus size" tire option against your factory size.
- Verifying that a spare or replacement tire size is close enough to the original to be safe.
FAQs
How much tire size difference is considered safe?
A common rule of thumb among tire shops is to keep the overall diameter difference within about 3% of the original size, to avoid meaningfully affecting speedometer accuracy, gearing, and ABS/traction control systems that rely on wheel speed sensors.
Why does my speedometer read wrong after changing tire size?
A speedometer is calibrated assuming a specific tire diameter and the resulting wheel rotations per mile. Fitting a larger-diameter tire means each rotation covers more real-world distance than the speedometer assumes, so it under-reads your actual speed — a smaller tire has the opposite effect and causes the speedometer to over-read.
What does the tire size notation mean?
In "205/55R16": 205 is the tire's width in millimeters, 55 is the aspect ratio (sidewall height as a percentage of that width), "R" indicates radial construction, and 16 is the wheel rim diameter in inches that the tire is designed to fit.