Tire Fitment Calculator
Use this calculator to compare your current tire size with a potential new size. It helps you understand how changes in tire dimensions affect overall diameter, sidewall height, and speedometer accuracy, ensuring proper fitment and performance.
Understanding Tire Fitment
Changing your vehicle's tire size can significantly impact its performance, appearance, and even safety. A tire fitment calculator helps you compare different tire dimensions to understand how they will affect your vehicle. This is crucial for maintaining speedometer accuracy, ensuring proper fender clearance, and preserving handling characteristics.
How to Read Tire Sizes (e.g., 205/55R16)
- 205: Section Width (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 steering feel and fuel economy.
- 55: Aspect Ratio (%) – This number represents the height of the tire's sidewall as a percentage of its section width. A lower aspect ratio (e.g., 45) means a shorter sidewall, often found on performance tires, which can improve handling but reduce ride comfort. A higher aspect ratio (e.g., 70) means a taller sidewall, common on trucks and SUVs, offering a smoother ride and better off-road capability.
- R: Construction Type – 'R' stands for Radial, the most common type of tire construction today.
- 16: Wheel Diameter (inches) – This is the diameter of the wheel rim in inches that the tire is designed to fit.
Key Calculations Explained
- Sidewall Height: This is the actual height of the tire's sidewall. It's calculated by multiplying the section width by the aspect ratio. A taller sidewall provides more cushioning, while a shorter one offers a more direct road feel.
- Overall Diameter: This is the total height of the tire when mounted on the wheel. It's the sum of the wheel diameter and twice the sidewall height. Changes in overall diameter directly impact speedometer accuracy and fender clearance.
- Revolutions Per Mile: This indicates how many times the tire rotates to cover one mile. A larger overall diameter means fewer revolutions per mile.
- Speedometer Difference: This is a critical output. If your new tires have a different overall diameter than your old ones, your speedometer will read inaccurately. For example, if your new tires are larger, your speedometer will read slower than your actual speed. A difference of more than 3% is generally not recommended without recalibrating your speedometer.
Considerations When Changing Tire Sizes
- Fender Clearance: Ensure the new tire's overall diameter and width will not rub against your vehicle's fenders, suspension components, or brake calipers, especially during turns or when the suspension is compressed.
- Speedometer Accuracy: A significant change in overall diameter will cause your speedometer to be inaccurate. This can lead to speeding tickets or misjudging distances. Recalibration may be necessary.
- Handling and Ride Quality: Wider tires can improve grip but may increase steering effort. Lower aspect ratio tires (shorter sidewalls) generally offer sharper handling but a harsher ride.
- Load Capacity: Ensure the new tires have an adequate load index for your vehicle's weight.
- TPMS (Tire Pressure Monitoring System): If your vehicle has TPMS, ensure the new wheels and tires are compatible.
- Aesthetics: While not performance-related, the visual impact of new wheels and tires is often a factor.
Always consult your vehicle's manufacturer recommendations or a professional tire specialist before making significant changes to your tire size.
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function calculateTireFitment() {
// Get current tire inputs
var currentSectionWidth = parseFloat(document.getElementById('currentSectionWidth').value);
var currentAspectRatio = parseFloat(document.getElementById('currentAspectRatio').value);
var currentWheelDiameter = parseFloat(document.getElementById('currentWheelDiameter').value);
// Get new tire inputs
var newSectionWidth = parseFloat(document.getElementById('newSectionWidth').value);
var newAspectRatio = parseFloat(document.getElementById('newAspectRatio').value);
var newWheelDiameter = parseFloat(document.getElementById('newWheelDiameter').value);
var resultDiv = document.getElementById('tireFitmentResult');
resultDiv.innerHTML = "; // Clear previous results
// Validate inputs
if (isNaN(currentSectionWidth) || isNaN(currentAspectRatio) || isNaN(currentWheelDiameter) ||
isNaN(newSectionWidth) || isNaN(newAspectRatio) || isNaN(newWheelDiameter) ||
currentSectionWidth <= 0 || currentAspectRatio <= 0 || currentWheelDiameter <= 0 ||
newSectionWidth <= 0 || newAspectRatio <= 0 || newWheelDiameter <= 0) {
resultDiv.innerHTML = 'Please enter valid positive numbers for all tire dimensions.';
return;
}
// — Calculations for Current Tire —
// Sidewall Height (inches) = (Section Width * Aspect Ratio / 100) / 25.4
var currentSidewallHeight = (currentSectionWidth * (currentAspectRatio / 100)) / 25.4;
// Overall Diameter (inches) = Wheel Diameter + (2 * Sidewall Height)
var currentOverallDiameter = currentWheelDiameter + (2 * currentSidewallHeight);
// Revolutions Per Mile = 63360 / (Overall Diameter * PI) (63360 inches in a mile)
var currentRevsPerMile = 63360 / (currentOverallDiameter * Math.PI);
// — Calculations for New Tire —
var newSidewallHeight = (newSectionWidth * (newAspectRatio / 100)) / 25.4;
var newOverallDiameter = newWheelDiameter + (2 * newSidewallHeight);
var newRevsPerMile = 63360 / (newOverallDiameter * Math.PI);
// — Comparison Calculations —
var diameterDifference = newOverallDiameter – currentOverallDiameter;
var diameterPercentageDifference = ((diameterDifference / currentOverallDiameter) * 100);
// Speedometer Reading (if old speed was 60 mph)
var currentSpeedometerReading = 60; // Example speed
var newActualSpeed = currentSpeedometerReading * (newOverallDiameter / currentOverallDiameter);
var speedometerError = newActualSpeed – currentSpeedometerReading;
// Display Results
var htmlOutput = '
Tire Fitment Comparison Results
';
htmlOutput += '
';
htmlOutput += '| Metric | Current Tire | New Tire | Difference |
|---|
';
htmlOutput += '';
htmlOutput += '| Section Width | ' + currentSectionWidth.toFixed(0) + ' mm | ' + newSectionWidth.toFixed(0) + ' mm | ' + (newSectionWidth – currentSectionWidth).toFixed(0) + ' mm |
';
htmlOutput += '| Aspect Ratio | ' + currentAspectRatio.toFixed(0) + ' % | ' + newAspectRatio.toFixed(0) + ' % | ' + (newAspectRatio – currentAspectRatio).toFixed(0) + ' % |
';
htmlOutput += '| Wheel Diameter | ' + currentWheelDiameter.toFixed(0) + ' inches | ' + newWheelDiameter.toFixed(0) + ' inches | ' + (newWheelDiameter – currentWheelDiameter).toFixed(0) + ' inches |
';
htmlOutput += '| Sidewall Height | ' + currentSidewallHeight.toFixed(2) + ' inches | ' + newSidewallHeight.toFixed(2) + ' inches | ' + (newSidewallHeight – currentSidewallHeight).toFixed(2) + ' inches |
';
htmlOutput += '| Overall Diameter | ' + currentOverallDiameter.toFixed(2) + ' inches | ' + newOverallDiameter.toFixed(2) + ' inches | ' + diameterDifference.toFixed(2) + ' inches |
';
htmlOutput += '| Revolutions Per Mile | ' + currentRevsPerMile.toFixed(0) + ' | ' + newRevsPerMile.toFixed(0) + ' | ' + (newRevsPerMile – currentRevsPerMile).toFixed(0) + ' |
';
htmlOutput += '';
htmlOutput += '
';
htmlOutput += '
Overall Diameter Difference: ' + diameterPercentageDifference.toFixed(2) + '%';
if (Math.abs(diameterPercentageDifference) > 3) {
htmlOutput += '
Warning: A ' + diameterPercentageDifference.toFixed(2) + '% difference in overall diameter is significant. Your speedometer will be inaccurate, and fitment issues are likely. Consider recalibration or a different tire size.';
} else if (Math.abs(diameterPercentageDifference) > 1) {
htmlOutput += '
Note: A ' + diameterPercentageDifference.toFixed(2) + '% difference in overall diameter might cause minor speedometer inaccuracy. Monitor for potential fitment issues.';
} else {
htmlOutput += '
Recommendation: A ' + diameterPercentageDifference.toFixed(2) + '% difference in overall diameter is generally acceptable for speedometer accuracy and fitment.';
}
htmlOutput += 'If your speedometer reads
' + currentSpeedometerReading + ' mph with your current tires, it will actually be reading
' + currentSpeedometerReading.toFixed(0) + ' mph when you are actually traveling at
' + newActualSpeed.toFixed(1) + ' mph with the new tires.';
htmlOutput += 'This means your speedometer will be off by approximately
' + speedometerError.toFixed(1) + ' mph (reads ' + (speedometerError > 0 ? 'slower' : 'faster') + ' than actual speed).';
resultDiv.innerHTML = htmlOutput;
}