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Tire Size Comparison Calculator

Compare two tire sizes and see the speedometer impact

Tire 1 (Original)

Tire 2 (New)

Metric Tire 1 Tire 2 Difference

Speedometer Error Result:

How to Read Tire Size Specifications

Changing your tire size affects your car's performance, fuel economy, and speedometer accuracy. Using a Tire Size Comparison Calculator is essential before upgrading your wheels. Here is what the numbers mean:

  • Width: The first number (e.g., 225) is the width of the tire in millimeters from sidewall to sidewall.
  • Aspect Ratio: The second number (e.g., 45) is the height of the sidewall expressed as a percentage of the width.
  • Wheel Diameter: The last number (e.g., 17) is the diameter of the wheel rim in inches.

Understanding Speedometer Error

If you install a tire with a larger overall diameter than your factory setup, your speedometer will display a speed lower than your actual velocity. This is because a larger tire covers more ground per revolution. Most mechanics recommend keeping the diameter difference within 3% of the original size to prevent issues with ABS, traction control, and transmission shifting points.

Example Calculation

Suppose you are switching from 225/45R17 to 245/40R18:

  • Tire 1 Diameter: (225 * 0.45 * 2 / 25.4) + 17 = 24.97 inches.
  • Tire 2 Diameter: (245 * 0.40 * 2 / 25.4) + 18 = 25.72 inches.
  • Difference: In this case, Tire 2 is 3% larger. When your speedometer reads 60 MPH, you are actually traveling 61.8 MPH.
function calculateTires() { // Inputs var t1w = parseFloat(document.getElementById('t1w').value); var t1r = parseFloat(document.getElementById('t1r').value); var t1d = parseFloat(document.getElementById('t1d').value); var t2w = parseFloat(document.getElementById('t2w').value); var t2r = parseFloat(document.getElementById('t2r').value); var t2d = parseFloat(document.getElementById('t2d').value); // Basic Validation if (isNaN(t1w) || isNaN(t1r) || isNaN(t1d) || isNaN(t2w) || isNaN(t2r) || isNaN(t2d)) { alert("Please enter valid numbers for all fields."); return; } // Tire 1 Logic var t1SidewallMm = t1w * (t1r / 100); var t1SidewallIn = t1SidewallMm / 25.4; var t1TotalDiam = (t1SidewallIn * 2) + t1d; var t1Circ = t1TotalDiam * Math.PI; var t1Revs = 63360 / t1Circ; // Revolutions per mile // Tire 2 Logic var t2SidewallMm = t2w * (t2r / 100); var t2SidewallIn = t2SidewallMm / 25.4; var t2TotalDiam = (t2SidewallIn * 2) + t2d; var t2Circ = t2TotalDiam * Math.PI; var t2Revs = 63360 / t2Circ; // Differences var diamDiff = t2TotalDiam – t1TotalDiam; var diamPerc = (diamDiff / t1TotalDiam) * 100; var speedAt60 = 60 * (t2TotalDiam / t1TotalDiam); // Build Table var tableHtml = "; tableHtml += createRow('Sidewall Height', t1SidewallIn.toFixed(2) + '"', t2SidewallIn.toFixed(2) + '"', (t2SidewallIn – t1SidewallIn).toFixed(2) + '"'); tableHtml += createRow('Total Diameter', t1TotalDiam.toFixed(2) + '"', t2TotalDiam.toFixed(2) + '"', diamDiff.toFixed(2) + '"'); tableHtml += createRow('Circumference', t1Circ.toFixed(2) + '"', t2Circ.toFixed(2) + '"', (t2Circ – t1Circ).toFixed(2) + '"'); tableHtml += createRow('Revs per Mile', t1Revs.toFixed(0), t2Revs.toFixed(0), (t2Revs – t1Revs).toFixed(0)); document.getElementById('comparison-table').innerHTML = tableHtml; // Result Text var resultMsg = "When your speedometer reads 60 MPH, you are actually traveling " + speedAt60.toFixed(1) + " MPH. "; resultMsg += "The total diameter difference is " + diamPerc.toFixed(2) + "%."; document.getElementById('speedo-result').innerHTML = resultMsg; // Safety Alert var alertBox = document.getElementById('alert-box'); if (Math.abs(diamPerc) > 3) { alertBox.style.display = "block"; alertBox.style.backgroundColor = "#fff3e0"; alertBox.style.color = "#e65100"; alertBox.style.border = "1px solid #ffb74d"; alertBox.innerHTML = "Caution: The diameter difference is " + diamPerc.toFixed(1) + "%. It is generally recommended to keep changes within 3% to avoid vehicle computer and safety system errors."; } else { alertBox.style.display = "block"; alertBox.style.backgroundColor = "#e8f5e9"; alertBox.style.color = "#2e7d32"; alertBox.style.border = "1px solid #a5d6a7"; alertBox.innerHTML = "Safe Fitment: The diameter difference is within the recommended 3% range."; } document.getElementById('results-area').style.display = 'block'; } function createRow(label, v1, v2, diff) { return '' + '' + label + '' + '' + v1 + '' + '' + v2 + '' + '' + diff + '' + ''; }

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