Tire Size Conversion Calculator
Original Tire Size
New Tire Size
Conversion Results
Original Tire Metrics:
Sidewall Height: 0 mm
Overall Diameter: 0 mm
Overall Circumference: 0 mm
New Tire Metrics:
Sidewall Height: 0 mm
Overall Diameter: 0 mm
Overall Circumference: 0 mm
Comparison:
Diameter Difference: 0 mm (0%)
Circumference Difference: 0 mm (0%)
Speedometer Error: 0%
Note: A positive speedometer error means your speedometer will read lower than your actual speed. A negative error means it will read higher.
Please enter valid positive numbers for all tire dimensions.
'; return; } // Calculate Original Tire Metrics var originalSidewall = (originalTreadWidth * (originalAspectRatio / 100)); var originalOverallDiameter = (originalSidewall * 2) + (originalRimDiameter * INCH_TO_MM); var originalCircumference = originalOverallDiameter * PI; // Calculate New Tire Metrics var newSidewall = (newTreadWidth * (newAspectRatio / 100)); var newOverallDiameter = (newSidewall * 2) + (newRimDiameter * INCH_TO_MM); var newCircumference = newOverallDiameter * PI; // Calculate Differences var diameterDifferenceMM = newOverallDiameter – originalOverallDiameter; var circumferenceDifferenceMM = newCircumference – originalCircumference; var diameterDifferencePercent = 0; var circumferenceDifferencePercent = 0; var speedometerError = 0; if (originalOverallDiameter !== 0) { diameterDifferencePercent = ((newOverallDiameter – originalOverallDiameter) / originalOverallDiameter) * 100; speedometerError = diameterDifferencePercent; // Speedometer error is directly related to diameter change } else { diameterDifferencePercent = NaN; // Indicate cannot calculate speedometerError = NaN; } if (originalCircumference !== 0) { circumferenceDifferencePercent = ((newCircumference – originalCircumference) / originalCircumference) * 100; } else { circumferenceDifferencePercent = NaN; // Indicate cannot calculate } // Display Results document.getElementById('originalSidewall').innerText = originalSidewall.toFixed(2) + ' mm'; document.getElementById('originalOverallDiameter').innerText = originalOverallDiameter.toFixed(2) + ' mm'; document.getElementById('originalCircumference').innerText = originalCircumference.toFixed(2) + ' mm'; document.getElementById('newSidewall').innerText = newSidewall.toFixed(2) + ' mm'; document.getElementById('newOverallDiameter').innerText = newOverallDiameter.toFixed(2) + ' mm'; document.getElementById('newCircumference').innerText = newCircumference.toFixed(2) + ' mm'; document.getElementById('diameterDifferenceMM').innerText = diameterDifferenceMM.toFixed(2) + ' mm'; document.getElementById('diameterDifferencePercent').innerText = (isNaN(diameterDifferencePercent) ? 'N/A' : diameterDifferencePercent.toFixed(2) + '%'); document.getElementById('circumferenceDifferenceMM').innerText = circumferenceDifferenceMM.toFixed(2) + ' mm'; document.getElementById('circumferenceDifferencePercent').innerText = (isNaN(circumferenceDifferencePercent) ? 'N/A' : circumferenceDifferencePercent.toFixed(2) + '%'); document.getElementById('speedometerError').innerText = (isNaN(speedometerError) ? 'N/A' : speedometerError.toFixed(2) + '%'); } // Run calculation on page load with default values window.onload = calculateTireConversion;Understanding Tire Size Conversion
Changing your vehicle's tire size can have significant implications for its performance, appearance, and even safety. Whether you're upgrading your wheels, seeking better handling, or simply replacing worn-out tires with a different specification, understanding how tire dimensions affect your vehicle is crucial. Our Tire Size Conversion Calculator helps you compare your original tire specifications with a potential new size, providing key metrics like overall diameter, circumference, and the critical speedometer error.
How to Read Tire Sizes (e.g., P205/55R16)
Before using the calculator, it's important to know how to interpret the numbers on your tire's sidewall:
- 205 (Tread Width in mm): This is the width of the tire's tread in millimeters, measured from sidewall to sidewall. A wider tire generally offers more grip but can also increase rolling resistance.
- 55 (Aspect Ratio %): This number represents the sidewall height as a percentage of the tread width. In this example, the sidewall height is 55% of 205mm. A lower aspect ratio means a shorter sidewall, often found on performance tires, providing better handling but a harsher ride.
- R (Construction Type): 'R' stands for Radial, which is the most common type of tire construction today.
- 16 (Rim Diameter in inches): This indicates the diameter of the wheel rim that the tire is designed to fit, measured in inches.
Key Metrics Explained
Our calculator provides several important metrics:
- Sidewall Height (mm): This is the vertical distance from the wheel rim to the top of the tread. It directly impacts ride comfort and the tire's ability to absorb impacts.
- Overall Diameter (mm): The total height of the tire from the ground to the top. This is a critical measurement as it affects ground clearance, gearing, and speedometer accuracy.
- Overall Circumference (mm): The distance covered by the tire in one complete rotation. This is directly proportional to the overall diameter and also impacts speedometer readings.
- Diameter Difference (mm and %): The absolute and percentage difference between the original and new tire's overall diameter. A difference of more than 3% is generally not recommended without professional advice, as it can significantly affect vehicle dynamics and safety systems.
- Circumference Difference (mm and %): Similar to diameter, this shows how much more or less distance the new tire covers per rotation.
- Speedometer Error (%): This is perhaps the most crucial output. If your new tires have a larger diameter, your speedometer will read lower than your actual speed. If they have a smaller diameter, it will read higher. A significant error can lead to speeding tickets or even dangerous situations if you misjudge your speed.
Why Tire Size Conversion Matters
Changing tire sizes without proper consideration can lead to several issues:
- Speedometer and Odometer Inaccuracy: As calculated, a change in overall diameter directly impacts how your vehicle's speed and distance traveled are measured.
- ABS/Traction Control Malfunctions: Modern vehicles rely on precise wheel speed sensor readings. Significant changes in tire diameter can confuse these systems, leading to erratic behavior or error codes.
- Rubbing and Clearance Issues: Larger tires might rub against fender wells, suspension components, or brake lines, especially during turns or when the suspension compresses.
- Altered Ride Quality and Handling: A lower aspect ratio (shorter sidewall) can improve handling responsiveness but often at the cost of a harsher ride. A taller sidewall can improve comfort but might feel less precise.
- Gearing and Fuel Economy: A larger overall diameter effectively changes your vehicle's final drive ratio, potentially impacting acceleration and fuel efficiency.
Example Calculation:
Let's use the default values in the calculator:
Original Tire: 205/55R16
- Tread Width: 205 mm
- Aspect Ratio: 55%
- Rim Diameter: 16 inches
- Calculated Sidewall Height: 205 * (55/100) = 112.75 mm
- Calculated Overall Diameter: (112.75 * 2) + (16 * 25.4) = 225.5 + 406.4 = 631.9 mm
- Calculated Overall Circumference: 631.9 * π ≈ 1985.1 mm
New Tire: 225/50R17
- Tread Width: 225 mm
- Aspect Ratio: 50%
- Rim Diameter: 17 inches
- Calculated Sidewall Height: 225 * (50/100) = 112.5 mm
- Calculated Overall Diameter: (112.5 * 2) + (17 * 25.4) = 225 + 431.8 = 656.8 mm
- Calculated Overall Circumference: 656.8 * π ≈ 2063.9 mm
Comparison:
- Diameter Difference: 656.8 – 631.9 = 24.9 mm
- Diameter Difference Percentage: (24.9 / 631.9) * 100 ≈ 3.94%
- Speedometer Error: Approximately +3.94%. If your speedometer reads 60 mph, your actual speed would be closer to 62.36 mph (60 * 1.0394).
In this example, the new tire is significantly larger, resulting in a speedometer error of nearly 4%. This is often considered too large for a safe and compliant tire change without recalibration.
Always consult with a professional tire specialist or your vehicle manufacturer's guidelines before making significant changes to your tire size.