Tire Size Calculator & Comparison
Enter a metric tire size to calculate overall diameter, sidewall height, circumference, and revolutions per mile. Add a second tire to compare side by side and see exact speedometer differences.
Enter a metric tire size in the format
Width / Aspect R Rim — for example 225/45R17 means 225mm wide, 45% aspect ratio, on a 17" rim.
Original Tire
/
R
Compare Tire (optional)
/
R
Original
205/55R16
—
diameter diff
Compare
215/55R16
Specifications
| Original | Compare |
|---|
Common Tire Sizes — Quick Reference
| Tire Size | Common Vehicle | Diameter | Load |
|---|---|---|---|
| 205/55R16 | Honda Civic, CR-V | 24.9" | Standard |
| 215/55R17 | Honda CR-V, HR-V | 26.3" | Standard |
| 225/45R17 | BMW 3 Series, Civic Si | 24.9" | Performance |
| 255/40R18 | BMW M3 (E9X front) | 25.4" | Performance |
| 265/35R18 | BMW M3 (E9X rear) | 25.3" | Performance |
| 235/55R18 | BMW X5, Honda CR-V | 28.2" | SUV |
| 245/50R20 | BMW X5 (F15) | 29.7" | SUV |
Need tires or wheels for your vehicle? Talk to our team at Iconic Garage — we can point you in the right direction for BMWs, Hondas, and more.
How Metric Tire Sizes Work
A metric tire size like 225/45R17 breaks down as: 225 = section width in millimeters, 45 = aspect ratio (sidewall height as a percentage of width), and 17 = rim diameter in inches. So this tire has a 225mm-wide tread, a sidewall height of 101.25mm (225 × 0.45), mounted on a 17-inch rim.
Overall tire diameter is calculated as: (rim diameter × 25.4) + (2 × sidewall height). This determines how the tire fits in the wheel well, how your speedometer reads, and how the vehicle's gearing effectively changes.
Why Tire Diameter Matters
Your vehicle's speedometer and odometer are calibrated for a specific tire diameter. Installing a tire that's larger or smaller changes how many rotations occur per mile — which shifts your actual speed vs. what the speedometer reads. It also affects your effective gear ratio: a taller tire makes acceleration slightly softer but can improve highway fuel economy.
Frequently Asked Questions
What is the aspect ratio on a tire? +
The aspect ratio is the sidewall height expressed as a percentage of the tire's section width. A 225/45R17 tire has a sidewall that is 45% of 225mm = 101.25mm tall. A lower aspect ratio (e.g., 35) means a shorter, stiffer sidewall — common on performance cars. A higher ratio (e.g., 65) means a taller sidewall — common on SUVs and vehicles needing more cushion.
How much diameter difference is acceptable? +
Most manufacturers recommend staying within ±3% of the stock tire diameter. Beyond that, you may see significant speedometer error, possible interference with wheel well components, and ABS/traction control calibration issues. A 1–2% difference is generally considered safe and barely noticeable.
Can I go wider without changing the rim? +
Yes, within limits. A tire can be safely mounted on a rim that is 1–2 inches narrower than the tire's section width. Going too wide or too narrow affects handling, wear, and safety. As a rule of thumb, a 225mm tire fits best on a 7–8" wide rim; a 255mm tire fits best on an 8.5–9.5" rim.
Why is my speedometer wrong after changing tire sizes? +
Speedometers count wheel rotations and convert that to speed using the stock tire's circumference. A larger tire has a bigger circumference — it covers more ground per rotation — so the speedometer reads lower than your actual speed. A smaller tire reads higher than actual. This calculator's speedometer table shows the exact difference at common speeds.
What does "plus sizing" mean? +
Plus sizing means going to a larger rim diameter while decreasing the tire's aspect ratio to maintain the same overall diameter. For example, replacing a 205/60R16 with a 225/50R17 keeps a similar diameter but puts the tire on a 17" rim. This improves handling (shorter sidewall = less flex) while keeping speedometer accuracy.
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