Tire Size Calculator (Compare Two Sizes)
Enter width (mm), aspect ratio (%), and wheel diameter (inches) for your current and new tire sizes.
What a tire size calculator helps you understand
Changing tire size affects far more than appearance. A larger or smaller overall diameter changes your speedometer reading, final drive ratio, ride height, and even how your transmission behaves. This tire size calculator helps you compare two metric tire sizes and quickly see the real-world impact before you buy wheels or tires.
For example, switching from 225/65R17 to 235/65R17 increases overall diameter and circumference. That means each wheel rotation covers more distance, so your vehicle may travel slightly faster than the speed shown on your dashboard.
How to read a tire size (example: 225/45R17)
- 225 = tire width in millimeters.
- 45 = aspect ratio (sidewall height is 45% of width).
- R = radial construction.
- 17 = wheel diameter in inches.
Core formulas used in this calculator
- Sidewall height (mm) = width × (aspect ratio ÷ 100)
- Sidewall height (in) = sidewall height (mm) ÷ 25.4
- Overall diameter (in) = wheel diameter + 2 × sidewall height (in)
- Circumference (in) = π × overall diameter
- Revolutions per mile = 63,360 ÷ circumference
Why diameter difference matters
1) Speedometer accuracy
If your new tire has a larger diameter than stock, your speedometer usually reads lower than your true speed. If it has a smaller diameter, your speedometer reads higher. Even a 2–3% difference is noticeable in daily driving.
2) Ground clearance and fitment
Half of the diameter change affects ride height. A tire that is 1 inch taller increases clearance by about 0.5 inches, but also comes closer to fenders, liners, struts, and suspension links. Always verify clearance at full steering lock and suspension compression.
3) Effective gearing and acceleration
Bigger tires act like taller gearing: lower RPM at a given speed, sometimes better highway comfort, but often softer low-speed acceleration. Smaller tires have the opposite effect. This is especially important for towing, off-road setups, and manual transmissions.
Practical guidelines when upsizing or downsizing
- Try to keep overall diameter within ±3% of OEM for street use.
- Confirm wheel width compatibility with the tire manufacturer’s spec sheet.
- Check load index and speed rating—never go below manufacturer requirements.
- On AWD vehicles, keep tire diameters closely matched across all four corners.
- After major changes, consider recalibrating speedometer and ADAS systems if required.
Common mistakes to avoid
- Choosing by looks only without checking total diameter difference.
- Ignoring tire weight, which can affect braking and fuel economy.
- Mixing very different rolling diameters on AWD/4WD systems.
- Forgetting to verify spare tire compatibility.
Quick FAQ
Is wider always better?
Not always. Wider tires can improve grip in some conditions, but may hydroplane more easily and reduce fuel efficiency. Correct width depends on wheel size, suspension geometry, and usage.
How much diameter change is “safe”?
A common rule is within ±3% of original diameter, but exact limits depend on your vehicle, local regulations, and whether recalibration is available.
Will changing tire size void warranty?
It depends on the issue and manufacturer policy. Significant non-OEM changes can affect claims tied to drivetrain, suspension, ABS, or electronic systems.
Disclaimer: This tool provides estimates for planning purposes. Always verify fitment with manufacturer data and a qualified tire professional.