Tyre Size Comparison Calculator

by

Tyre Size Comparison Calculator

Original Tyre

New Tyre

Comparison Results

Original Tyre Details:

Size:

Sidewall Height: mm

Overall Diameter: mm ( inches)

Circumference: mm

Revolutions per km:

Revolutions per mile:

New Tyre Details:

Size:

Sidewall Height: mm

Overall Diameter: mm ( inches)

Circumference: mm

Revolutions per km:

Revolutions per mile:

Comparison Summary:

Overall Diameter Difference: mm (%)

Sidewall Height Difference: mm (%)

Speedometer Error: % (When your speedometer reads 100 km/h, actual speed will be km/h)

Note: A speedometer error of more than +/- 3% is generally not recommended.

function calculateTyreComparison() { var originalWidth = parseFloat(document.getElementById('originalWidth').value); var originalAspect = parseFloat(document.getElementById('originalAspect').value); var originalRim = parseFloat(document.getElementById('originalRim').value); var newWidth = parseFloat(document.getElementById('newWidth').value); var newAspect = parseFloat(document.getElementById('newAspect').value); var newRim = parseFloat(document.getElementById('newRim').value); // Input validation if (isNaN(originalWidth) || isNaN(originalAspect) || isNaN(originalRim) || isNaN(newWidth) || isNaN(newAspect) || isNaN(newRim) || originalWidth <= 0 || originalAspect <= 0 || originalRim <= 0 || newWidth <= 0 || newAspect <= 0 || newRim <= 0) { document.getElementById('tyreComparisonResult').innerHTML = 'Please enter valid positive numbers for all tyre dimensions.'; return; } var PI = Math.PI; var INCH_TO_MM = 25.4; var KM_TO_MM = 1000000; // 1 km = 1,000,000 mm var MILE_TO_MM = 1609344; // 1 mile = 1,609,344 mm // — Calculations for Original Tyre — var originalSidewallHeight = (originalWidth * (originalAspect / 100)); var originalRimDiameterMM = originalRim * INCH_TO_MM; var originalOverallDiameterMM = originalRimDiameterMM + (2 * originalSidewallHeight); var originalOverallDiameterInches = originalOverallDiameterMM / INCH_TO_MM; var originalCircumference = originalOverallDiameterMM * PI; var originalRevsPerKm = KM_TO_MM / originalCircumference; var originalRevsPerMile = MILE_TO_MM / originalCircumference; // — Calculations for New Tyre — var newSidewallHeight = (newWidth * (newAspect / 100)); var newRimDiameterMM = newRim * INCH_TO_MM; var newOverallDiameterMM = newRimDiameterMM + (2 * newSidewallHeight); var newOverallDiameterInches = newOverallDiameterMM / INCH_TO_MM; var newCircumference = newOverallDiameterMM * PI; var newRevsPerKm = KM_TO_MM / newCircumference; var newRevsPerMile = MILE_TO_MM / newCircumference; // — Comparison Calculations — var diameterDifferenceMM = newOverallDiameterMM – originalOverallDiameterMM; var diameterDifferencePercent = (diameterDifferenceMM / originalOverallDiameterMM) * 100; var sidewallDifferenceMM = newSidewallHeight – originalSidewallHeight; var sidewallDifferencePercent = (sidewallDifferenceMM / originalSidewallHeight) * 100; var speedometerError = ((newOverallDiameterMM – originalOverallDiameterMM) / originalOverallDiameterMM) * 100; var actualSpeedAt100 = 100 * (1 + (speedometerError / 100)); // — Display Results — document.getElementById('originalSizeDisplay').innerHTML = originalWidth + '/' + originalAspect + ' R' + originalRim; document.getElementById('originalSidewall').innerHTML = originalSidewallHeight.toFixed(2); document.getElementById('originalOverallDiameterMM').innerHTML = originalOverallDiameterMM.toFixed(2); document.getElementById('originalOverallDiameterInches').innerHTML = originalOverallDiameterInches.toFixed(2); document.getElementById('originalCircumference').innerHTML = originalCircumference.toFixed(2); document.getElementById('originalRevsKm').innerHTML = originalRevsPerKm.toFixed(2); document.getElementById('originalRevsMile').innerHTML = originalRevsPerMile.toFixed(2); document.getElementById('newSizeDisplay').innerHTML = newWidth + '/' + newAspect + ' R' + newRim; document.getElementById('newSidewall').innerHTML = newSidewallHeight.toFixed(2); document.getElementById('newOverallDiameterMM').innerHTML = newOverallDiameterMM.toFixed(2); document.getElementById('newOverallDiameterInches').innerHTML = newOverallDiameterInches.toFixed(2); document.getElementById('newCircumference').innerHTML = newCircumference.toFixed(2); document.getElementById('newRevsKm').innerHTML = newRevsPerKm.toFixed(2); document.getElementById('newRevsMile').innerHTML = newRevsPerMile.toFixed(2); document.getElementById('diameterDiffMM').innerHTML = diameterDifferenceMM.toFixed(2); document.getElementById('diameterDiffPercent').innerHTML = diameterDifferencePercent.toFixed(2); document.getElementById('sidewallDiffMM').innerHTML = sidewallDifferenceMM.toFixed(2); document.getElementById('sidewallDiffPercent').innerHTML = sidewallDifferencePercent.toFixed(2); document.getElementById('speedometerError').innerHTML = speedometerError.toFixed(2); document.getElementById('actualSpeed').innerHTML = actualSpeedAt100.toFixed(2); } // Run calculation on page load with default values window.onload = calculateTyreComparison;

Understanding Tyre Size Comparison

Changing your vehicle's tyre size can have significant implications for its performance, safety, and even legality. Our Tyre Size Comparison Calculator helps you understand these changes by comparing your original tyre specifications with a potential new size.

What Do Tyre Sizes Mean? (e.g., 205/55 R16)

  • 205 (Tyre Width in mm): This is the width of the tyre in millimeters, measured from sidewall to sidewall. A wider tyre generally offers more grip but can increase rolling resistance and potentially rub against wheel wells.
  • 55 (Aspect Ratio %): This number represents the sidewall height as a percentage of the tyre's width. In this example, the sidewall height is 55% of 205mm. A lower aspect ratio means a shorter sidewall, often found on performance tyres, offering better handling but a harsher ride.
  • R (Construction Type): 'R' stands for Radial, which is the most common type of tyre construction today.
  • 16 (Rim Diameter in inches): This is the diameter of the wheel rim that the tyre is designed to fit, measured in inches.

Why Compare Tyre Sizes?

There are several reasons why you might consider changing your tyre size, or why you need to compare sizes:

  • Speedometer Accuracy: The most critical factor. If the overall diameter of your new tyre is different from the original, your speedometer will read incorrectly. A larger diameter tyre will make your speedometer read lower than your actual speed, while a smaller diameter tyre will make it read higher.
  • Vehicle Fitment: Will the new tyre physically fit within your wheel wells and clear suspension components, especially when turning or under compression?
  • Handling and Ride Quality: Changes in sidewall height and overall diameter can affect how your car handles, its cornering ability, and the comfort of your ride.
  • Aesthetics: Many people change tyre sizes for a different look, often opting for larger rims and lower profile tyres.
  • Braking Performance: Significant changes in tyre diameter can affect braking dynamics.
  • Fuel Economy: Larger or wider tyres can increase rolling resistance, potentially impacting fuel efficiency.

Key Metrics Explained by the Calculator:

  • Sidewall Height: The vertical distance from the wheel rim to the top of the tread. This impacts ride comfort and handling.
  • Overall Diameter: The total height of the inflated tyre. This is crucial for speedometer accuracy and vehicle clearance.
  • Circumference: The distance covered by one full rotation of the tyre. Directly related to overall diameter.
  • Revolutions per km/mile: How many times the tyre rotates to cover one kilometer or mile. This is inversely proportional to the overall diameter.
  • Speedometer Error: The percentage difference in speed reading. A positive percentage means your actual speed is higher than what the speedometer shows, and a negative percentage means it's lower. For safety and legal reasons, it's generally recommended to keep this error within +/- 3%.

Important Considerations When Changing Tyre Sizes:

  1. Stay Within 3% Overall Diameter Difference: Most experts recommend keeping the overall diameter difference between your new and original tyres within +/- 3%. Exceeding this can lead to significant speedometer errors, affect ABS/ESP systems, and potentially cause transmission issues.
  2. Check for Clearance: Always ensure the new tyre size will not rub against fenders, suspension components, or brake calipers, especially when the steering wheel is at full lock or the suspension is compressed.
  3. Load Index and Speed Rating: Ensure the new tyres have a load index and speed rating equal to or greater than the vehicle manufacturer's recommendations.
  4. Local Regulations: Some regions have specific laws regarding tyre size modifications. Always check your local regulations.
  5. Professional Advice: When in doubt, consult with a professional tyre specialist or your vehicle manufacturer.

Use this calculator as a guide to make informed decisions about your tyre choices, ensuring both safety and optimal vehicle performance.

Leave a Comment