Tire Size Calculator
Decode tire sizes, compare side-by-side, and calculate speedometer correction, diameter and clearance changes instantly
Tire Size Format
Enter as 225/45R17 — Width/AspectR Rim
How to Use
- 1 Enter your current tire size in the first field (e.g. 225/45R17)
- 2 See the decoded dimensions instantly: diameter, width, sidewall height, circumference
- 3 Enter a new tire size in the second field to compare side-by-side
- 4 Check the speedometer error and compatibility rating before buying new tires
What You Get
A comprehensive tire size calculator that decodes metric tire sizes (e.g. 225/45R17) into real-world measurements. Compare two tire sizes side-by-side to see diameter difference, speedometer error, ground clearance change, and whether the new size is safe to use. Supports all metric (P-metric and Euro-metric) tire formats.
Input: 225/45R17
Output: Diameter: 634.3 mm (24.97"), Width: 225 mm, Circumference: 1993 mm
Input: 225/45R17 vs 235/40R18
Output: +1.72% speedometer error, +5.45 mm clearance change – ✅ Safe
Input: 265/70R17 vs 285/75R17
Output: +7.0% speedometer error – 🔴 Not recommended (>5%)
Input: P205/55R16
Output: Diameter: 631.9 mm (24.88"), Width: 205 mm, Circumference: 1985 mm
What do the numbers on a tire mean?
A tire size like 225/45R17 breaks down as: 225 = tread width in millimeters, 45 = aspect ratio (sidewall height is 45% of the width), R = radial construction, 17 = wheel diameter in inches. So the sidewall height is 225 × 0.45 = 101.25 mm.
How do I know what size tires fit my car?
Check the placard on your driver-side door jamb or your owner's manual for the OEM tire size. When upgrading, stay within 3% of the original overall diameter to avoid speedometer errors, clearance issues, and potential brake system interference.
Will bigger tires affect my speedometer?
Yes. Larger tires cover more distance per revolution, so your speedometer will read lower than your actual speed. For example, switching from 225/45R17 to 245/45R17 creates about a 2.8% error — at 100 km/h, you're actually doing ~103 km/h. Our calculator shows the exact speedometer error.
What does the R mean in tire size?
The R stands for "Radial" construction, which means the tire's internal cord plies radiate outward from the center. Nearly all modern passenger tires are radial. Older bias-ply tires used a "B" or "D" designation. Radial tires offer better fuel economy, tread wear, and ride comfort.
How much bigger can I go on tire size?
The industry standard is to stay within 3% of the original tire diameter. Going beyond 3% can cause speedometer inaccuracy, affect ABS and traction control systems, void warranty, and cause rubbing on fenders or suspension components. Our calculator color-codes the compatibility: green (<3%), yellow (3–5%), red (>5%).
What is the 3 percent rule for tire sizes?
The 3% rule means your new tire's overall diameter should be within 3% of the original equipment size. This keeps speedometer error minimal, maintains proper ABS and traction control function, and avoids clearance issues. For a tire with 630 mm diameter, you should stay between 611–649 mm.
Is this tire size calculator free to use?
Yes, 100% free with no signup required. All calculations happen directly in your browser — we don't store any data or require personal information.
How do I calculate tire diameter from tire size?
Formula: Diameter = Wheel diameter (in mm) + 2 × Sidewall height. Sidewall height = Width × (Aspect ratio / 100). For 225/45R17: Sidewall = 225 × 0.45 = 101.25 mm. Diameter = (17 × 25.4) + (2 × 101.25) = 431.8 + 202.5 = 634.3 mm.
100% client-side. All calculations happen in your browser – nothing is sent to any server.