Tire Calculator

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Tire Size Comparison Calculator

Compare dimensions, speedometer error, and fitment differences

Tire 1 (Stock)

Example: 215
Example: 65
Example: 15

Tire 2 (New)

Example: 225
Example: 70
Example: 16
Metric Tire 1 Tire 2 Difference

How to Use the Tire Size Calculator

Choosing the right tire size is critical for maintaining your vehicle's safety, handling, and speedometer accuracy. This calculator allows you to compare your stock (Original Equipment) tire measurements against a potential new tire size to see how the dimensions change.

Understanding Tire Metrics

  • Section Width: The width of the tire from sidewall to sidewall in millimeters.
  • Aspect Ratio: The height of the sidewall expressed as a percentage of the width. A 65 aspect ratio means the sidewall is 65% as tall as the tire is wide.
  • Wheel Diameter: The size of the wheel (rim) the tire is designed to fit, measured in inches.
  • Overall Diameter: The total height of the tire from the ground to the top.
  • Circumference: The distance around the outside of the tire. This determines how far the vehicle travels in one rotation.

The Importance of Speedometer Error

Your vehicle's speedometer is calibrated based on the factory tire diameter. If you install a taller tire (larger diameter), your speedometer will read slower than your actual speed because the tire covers more ground per revolution. Conversely, a shorter tire will result in a speedometer reading that is faster than your actual speed.

Pro Tip: Most automotive experts recommend staying within 3% of the original tire diameter to avoid issues with Anti-lock Braking Systems (ABS), Traction Control, and transmission shift points.

Calculation Example

If you move from a 215/65R15 to a 225/70R16:

  1. The sidewall height increases from 139.75mm to 157.5mm.
  2. The overall diameter jumps from 26.0 inches to 28.4 inches.
  3. When your speedometer reads 60 MPH, you are actually traveling approximately 65.5 MPH.
function calculateTireComparison() { var w1 = parseFloat(document.getElementById('w1').value); var ar1 = parseFloat(document.getElementById('ar1').value); var wd1 = parseFloat(document.getElementById('wd1').value); var w2 = parseFloat(document.getElementById('w2').value); var ar2 = parseFloat(document.getElementById('ar2').value); var wd2 = parseFloat(document.getElementById('wd2').value); if (isNaN(w1) || isNaN(ar1) || isNaN(wd1) || isNaN(w2) || isNaN(ar2) || isNaN(wd2)) { alert('Please enter valid numbers for all fields.'); return; } // Calculations for Tire 1 var sw1 = w1 * (ar1 / 100); // sidewall mm var dia1 = ((sw1 * 2) / 25.4) + wd1; // diameter inches var circ1 = dia1 * Math.PI; var rpm1 = 63360 / circ1; // revs per mile // Calculations for Tire 2 var sw2 = w2 * (ar2 / 100); // sidewall mm var dia2 = ((sw2 * 2) / 25.4) + wd2; // diameter inches var circ2 = dia2 * Math.PI; var rpm2 = 63360 / circ2; // revs per mile // Differences var diaDiff = ((dia2 – dia1) / dia1) * 100; var speedoReading = 60; var actualSpeed = speedoReading * (dia2 / dia1); var resultsBody = document.getElementById('results-body'); var rows = [ ['Section Width', w1 + ' mm', w2 + ' mm', (w2 – w1).toFixed(1) + ' mm'], ['Sidewall Height', (sw1 / 25.4).toFixed(2) + ' in', (sw2 / 25.4).toFixed(2) + ' in', ((sw2 – sw1) / 25.4).toFixed(2) + ' in'], ['Overall Diameter', dia1.toFixed(2) + ' in', dia2.toFixed(2) + ' in', (dia2 – dia1).toFixed(2) + ' in'], ['Circumference', circ1.toFixed(2) + ' in', circ2.toFixed(2) + ' in', (circ2 – circ1).toFixed(2) + ' in'], ['Revs per Mile', rpm1.toFixed(0), rpm2.toFixed(0), (rpm2 – rpm1).toFixed(0)], ['Diameter Difference', '-', '-', diaDiff.toFixed(2) + '%'] ]; var html = "; for (var i = 0; i < rows.length; i++) { html += ''; html += '' + rows[i][0] + ''; html += '' + rows[i][1] + ''; html += '' + rows[i][2] + ''; html += ' 0 ? "#e74c3c" : "#27ae60″) + ';">' + rows[i][3] + ''; html += ''; } resultsBody.innerHTML = html; var warningDiv = document.getElementById('speedo-warning'); var warningText = 'Speedometer Error: When your speedometer reads 60 MPH, you are actually going ' + actualSpeed.toFixed(1) + ' MPH.'; if (Math.abs(diaDiff) > 3) { warningDiv.style.background = '#fdeaea'; warningDiv.style.color = '#c0392b'; warningText += '
Warning: Difference is greater than 3%. Fitment and safety systems may be affected.'; } else { warningDiv.style.background = '#eafaf1'; warningDiv.style.color = '#27ae60'; warningText += '
Safe: Difference is within the recommended 3% range.'; } warningDiv.innerHTML = warningText; document.getElementById('tire-results').style.display = 'block'; }

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