Tiresize Calculator

Tire Size Comparison Calculator

Original Tire Specifications

New Tire Specifications

Comparison Results

function calculateTireSize() { // Get original tire inputs var originalWidth = parseFloat(document.getElementById('originalWidth').value); var originalAspect = parseFloat(document.getElementById('originalAspect').value); var originalRim = parseFloat(document.getElementById('originalRim').value); // Get new tire inputs var newWidth = parseFloat(document.getElementById('newWidth').value); var newAspect = parseFloat(document.getElementById('newAspect').value); var newRim = parseFloat(document.getElementById('newRim').value); // Validate inputs 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('tireResult').innerHTML = 'Please enter valid positive numbers for all tire specifications.'; return; } // Helper function to calculate tire metrics function getTireMetrics(width, aspect, rim) { var sidewallHeight = (width * (aspect / 100)); // mm var rimDiameterMM = rim * 25.4; // inches to mm var overallDiameter = (2 * sidewallHeight) + rimDiameterMM; // mm var circumference = overallDiameter * Math.PI; // mm var revolutionsPerMile = 1609344 / circumference; // 1 mile = 1609344 mm return { sidewallHeight: sidewallHeight, overallDiameter: overallDiameter, circumference: circumference, revolutionsPerMile: revolutionsPerMile }; } var originalMetrics = getTireMetrics(originalWidth, originalAspect, originalRim); var newMetrics = getTireMetrics(newWidth, newAspect, newRim); // Calculate differences var diameterDifference = newMetrics.overallDiameter – originalMetrics.overallDiameter; var circumferenceDifference = newMetrics.circumference – originalMetrics.circumference; var sidewallDifference = newMetrics.sidewallHeight – originalMetrics.sidewallHeight; var speedometerError = ((newMetrics.overallDiameter – originalMetrics.overallDiameter) / originalMetrics.overallDiameter) * 100; var revsPerMileDifference = newMetrics.revolutionsPerMile – originalMetrics.revolutionsPerMile; // Display results var originalSummaryHtml = `

Original Tire (${originalWidth}/${originalAspect}R${originalRim})

  • Sidewall Height: ${originalMetrics.sidewallHeight.toFixed(2)} mm
  • Overall Diameter: ${originalMetrics.overallDiameter.toFixed(2)} mm (${(originalMetrics.overallDiameter / 25.4).toFixed(2)} inches)
  • Circumference: ${originalMetrics.circumference.toFixed(2)} mm
  • Revolutions per Mile: ${originalMetrics.revolutionsPerMile.toFixed(2)}
`; var newSummaryHtml = `

New Tire (${newWidth}/${newAspect}R${newRim})

  • Sidewall Height: ${newMetrics.sidewallHeight.toFixed(2)} mm
  • Overall Diameter: ${newMetrics.overallDiameter.toFixed(2)} mm (${(newMetrics.overallDiameter / 25.4).toFixed(2)} inches)
  • Circumference: ${newMetrics.circumference.toFixed(2)} mm
  • Revolutions per Mile: ${newMetrics.revolutionsPerMile.toFixed(2)}
`; var comparisonHtml = `

Comparison Details

  • Diameter Difference: ${diameterDifference.toFixed(2)} mm (${(diameterDifference / 25.4).toFixed(2)} inches)
  • Circumference Difference: ${circumferenceDifference.toFixed(2)} mm
  • Sidewall Height Difference: ${sidewallDifference.toFixed(2)} mm
  • Revolutions per Mile Difference: ${revsPerMileDifference.toFixed(2)}
  • Speedometer Error: ${speedometerError.toFixed(2)}% ${speedometerError > 0 ? 'Your speedometer will read ' + Math.abs(speedometerError).toFixed(2) + '% SLOWER than actual speed.' : speedometerError < 0 ? 'Your speedometer will read ' + Math.abs(speedometerError).toFixed(2) + '% FASTER than actual speed.' : 'Your speedometer will be accurate.'}
`; document.getElementById('originalTireSummary').innerHTML = originalSummaryHtml; document.getElementById('newTireSummary').innerHTML = newSummaryHtml; document.getElementById('comparisonDetails').innerHTML = comparisonHtml; }

Understanding Your Tire Size and Why Comparison Matters

Choosing the right tire size for your vehicle is crucial for safety, performance, and even the accuracy of your speedometer. Whether you're upgrading your wheels, replacing worn-out tires, or simply curious about different options, understanding tire specifications is key. Our Tire Size Comparison Calculator helps you quickly compare two different tire sizes and understand the impact of changing them.

Decoding Tire Size Numbers (e.g., 205/55R16)

A typical tire size, like 205/55R16, contains three vital pieces of information:

  • 205 (Tire Width in mm): This is the width of the tire in millimeters, measured from sidewall to sidewall. A wider tire generally offers more grip but can affect fuel economy and steering feel.
  • 55 (Aspect Ratio %): This number represents the sidewall height as a percentage of the tire's 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.
  • R16 (Rim Diameter in inches): The 'R' stands for Radial construction (the most common type), and '16' indicates the diameter of the wheel (rim) in inches that the tire is designed to fit.

Why Compare Tire Sizes?

Changing your tire size, even slightly, can have several implications:

  • Speedometer Accuracy: This is one of the most critical factors. If your new tires have a different overall diameter than your original ones, your speedometer will no longer read accurately. A larger diameter tire will make your speedometer read slower than your actual speed, while a smaller diameter tire will make it read faster. This can lead to speeding tickets or misjudging distances.
  • Fitment Issues: Tires that are too large might rub against your vehicle's fenders, suspension components, or brake calipers, especially during turns or when the suspension compresses.
  • Handling and Ride Quality: Changes in sidewall height and tire width can significantly alter your vehicle's handling characteristics. Lower profile tires (smaller aspect ratio) generally offer sharper steering response but can result in a firmer ride.
  • Ground Clearance: A larger overall diameter will increase your vehicle's ground clearance, which can be beneficial for off-roading but might raise the center of gravity.
  • Braking Performance: While often subtle, changes in tire size can affect braking distances and ABS calibration.
  • Fuel Economy: Larger or wider tires can increase rolling resistance, potentially leading to a slight decrease in fuel efficiency.

How to Use the Calculator

Our calculator makes comparing tire sizes straightforward:

  1. Enter Original Tire Specs: Input the width, aspect ratio, and rim diameter of your current or original tires into the "Original Tire Specifications" section.
  2. Enter New Tire Specs: Input the width, aspect ratio, and rim diameter of the new tires you are considering into the "New Tire Specifications" section.
  3. Click Calculate: The calculator will instantly display detailed metrics for both tires, including sidewall height, overall diameter, circumference, and revolutions per mile.
  4. Review Comparison Details: Most importantly, it will show you the differences in these metrics and, crucially, the estimated speedometer error.

Example Scenario:

Let's say your original tires are 205/55R16, and you're considering upgrading to 225/45R17.

  • Original Tire (205/55R16):
    • Sidewall Height: (205 * 0.55) = 112.75 mm
    • Rim Diameter: 16 inches * 25.4 mm/inch = 406.4 mm
    • Overall Diameter: (2 * 112.75) + 406.4 = 631.9 mm
  • New Tire (225/45R17):
    • Sidewall Height: (225 * 0.45) = 101.25 mm
    • Rim Diameter: 17 inches * 25.4 mm/inch = 431.8 mm
    • Overall Diameter: (2 * 101.25) + 431.8 = 634.3 mm

In this example, the new tire has a slightly larger overall diameter (634.3 mm vs. 631.9 mm). This means your speedometer would read slightly slower than your actual speed. The calculator would show a small positive speedometer error percentage, indicating that when your speedometer reads 60 mph, you might actually be going 60.2 mph.

Always consult with a professional tire installer or your vehicle manufacturer's recommendations before making significant changes to your tire size to ensure safety and optimal performance.

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