A tire size calculator lets you compare two tire sizes side by side to understand exactly how they differ in diameter, width, sidewall height, and circumference. Whether you are upgrading wheels, plus-sizing your tires, or replacing with an alternative size, this tool shows you the physical differences and calculates the speedometer correction so you know your actual speed after a tire swap.
Popular Tire Sizes
Click a size to fill it into the currently selected tire slot (Original or New).
Original Tire
New Tire
Visual Size Comparison
Side-by-Side Comparison
| Measurement | Original | New | Difference |
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Speedometer Correction
When you change tire sizes, your speedometer may no longer be accurate. Here is how your speedometer reads compared to actual speed with the new tires:
Fitment Safety Check
The new tire diameter is within the recommended 3% range.
How to Use the Tire Size Calculator
Changing tire sizes affects more than just how your car looks. Different tire dimensions change your overall wheel diameter, which impacts speedometer accuracy, ride comfort, handling, and ground clearance. This tire size calculator makes it easy to compare two tire sizes so you can make an informed decision before purchasing new tires or wheels.
Step 1: Select Your Original Tire Size
Use the three dropdown menus under "Original Tire" to enter the width (in millimeters), aspect ratio (sidewall height as a percentage of width), and rim diameter (in inches) of your current tires. You can find your tire size printed on the sidewall of your existing tires, in the format like 225/45R17. It is also listed on the placard inside the driver's door jamb and in your vehicle owner's manual.
Step 2: Select Your New Tire Size
Enter the tire size you are considering under "New Tire." If you are plus-sizing to larger wheels, you will typically increase the rim diameter and decrease the aspect ratio to maintain a similar overall diameter. Use the popular tire sizes buttons at the top to quickly fill in common sizes.
Step 3: Compare the Results
Click "Compare Tire Sizes" to see a full side-by-side breakdown. The comparison table shows overall diameter, section width, sidewall height, circumference, and revolutions per mile for both tire sizes, along with the exact difference in millimeters, inches, and percentage. The visual diagram gives you an intuitive sense of the size difference at a glance.
Step 4: Check the Speedometer Correction
The speedometer correction section shows what your actual speed will be at various indicated speeds if you switch to the new tire size. A larger tire means your car covers more ground per revolution, so your speedometer will read slower than you are actually going. This is critical information for avoiding speeding tickets and maintaining accurate mileage tracking.
Understanding the Fitment Safety Check
The fitment gauge shows whether the overall diameter difference falls within the safe range. Most vehicle manufacturers recommend staying within 3% of the original tire diameter. Exceeding this threshold can affect ABS, traction control, and stability systems that rely on wheel speed sensors. The calculator clearly flags whether your proposed tire swap is safe, marginal, or potentially problematic.
Tire Size Math Explained
The tire size comparison formulas are straightforward. Sidewall height equals width multiplied by the aspect ratio divided by 100. Overall diameter is twice the sidewall height plus the rim diameter converted to millimeters (rim inches multiplied by 25.4). Circumference is pi times the overall diameter. Revolutions per mile equals 63,360 (inches in a mile) divided by circumference in inches. All calculations run instantly in your browser with no data sent to any server.
FAQ
Is this tire size calculator really free?
Yes, completely free with no signup required. All calculations run locally in your browser — no data is ever sent to a server. You can compare as many tire sizes as you want.
Is my data safe when using this tool?
Absolutely. Everything is calculated in your browser using JavaScript. No tire size data or personal information is transmitted or stored anywhere. Your privacy is fully protected.
What does a tire size like 225/45R17 mean?
The first number (225) is the tire width in millimeters. The second number (45) is the aspect ratio — the sidewall height as a percentage of the width. R means radial construction. The last number (17) is the rim diameter in inches. So a 225/45R17 tire is 225mm wide with a sidewall that is 45% of 225mm tall, on a 17-inch wheel.
How much tire size difference is safe?
Most manufacturers recommend staying within 3% of the original tire's overall diameter. Going beyond this can affect speedometer accuracy, ABS and traction control calibration, and may cause rubbing on wheel wells. Always check your vehicle's manual and consult a tire professional before making significant size changes.
How does tire size affect my speedometer?
Larger tires cover more ground per revolution, so your speedometer reads lower than your actual speed. Smaller tires cause it to read higher. For example, going from a 26-inch to a 27-inch overall diameter tire means at an indicated 60 mph, you are actually traveling about 62.3 mph.
What is plus sizing for tires?
Plus sizing means increasing the wheel diameter while decreasing the sidewall height to maintain approximately the same overall tire diameter. For example, going from 225/55R16 to 225/45R18. This improves handling and appearance while keeping the speedometer accurate and avoiding clearance issues.
How are revolutions per mile calculated?
Revolutions per mile equals 63,360 (inches in a mile) divided by the tire circumference in inches. A larger tire has a greater circumference, so it makes fewer revolutions per mile. This affects odometer accuracy and speedometer readings, and is important for gear ratio calculations.
Can I use this for metric rim sizes?
This calculator uses the standard tire notation format where width is in millimeters, aspect ratio is a percentage, and rim diameter is in inches. This is the most common format worldwide, used on virtually all passenger car and light truck tires regardless of country.