Enter your current and new tire dimensions to see the difference.
Understanding Tire Size and Its Impact
Choosing the right tire size is crucial for your vehicle's performance, safety, and even its speedometer and odometer accuracy. Tire sizes are expressed using a standardized code, typically found on the sidewall of the tire. This calculator helps you understand the physical differences between two tire sizes.
Decoding the Tire Size Code
A common tire size format looks like this: P205/55R16
P: Indicates a Passenger car tire. Other prefixes like LT (Light Truck) exist.
205: This is the tire width in millimeters (mm). It's the measurement from sidewall to sidewall.
55: This is the aspect ratio (or profile). It's the tire's sidewall height as a percentage of its width. In this case, the sidewall height is 55% of 205mm.
R: Indicates Radial construction.
16: This is the rim diameter in inches. It's the size of the wheel the tire is designed to fit.
How the Calculation Works
This calculator determines the key dimensions of each tire and then compares them:
Tire Section Width (mm): Directly provided by the user (e.g., 205mm).
Sidewall Height (mm): Calculated as (Tire Section Width * Aspect Ratio) / 100.
Example: For 205/55R16, Sidewall Height = (205 * 55) / 100 = 112.75 mm.
Overall Diameter (mm): Calculated as (Sidewall Height * 2) + (Rim Diameter in inches * 25.4). We multiply rim diameter by 25.4 to convert inches to millimeters.
Example: For 205/55R16, Overall Diameter = (112.75 * 2) + (16 * 25.4) = 225.5 + 406.4 = 631.9 mm.
Circumference (mm): Calculated as Overall Diameter * π (pi, approximately 3.14159).
Example: For 205/55R16, Circumference = 631.9 * π ≈ 1985.1 mm.
Revolutions Per Mile: Calculated as (63360 inches/mile) / (Overall Diameter in inches).
Example: For 205/55R16, Overall Diameter (inches) = 631.9 mm / 25.4 mm/inch ≈ 24.88 inches. Revolutions Per Mile = 63360 / 24.88 ≈ 2546.6.
Why This Matters
The calculator highlights differences in:
Overall Diameter: A larger diameter can slightly increase ride height and affect speedometer readings (making the car seem slower than it is). A smaller diameter can lower the vehicle and make it seem faster.
Section Width: Affects the tire's contact patch with the road, influencing grip, handling, and fuel efficiency.
Sidewall Height: Contributes to ride comfort and handling characteristics.
Speedometer Error: A significant difference in diameter directly translates to speedometer and odometer inaccuracies. For example, if the new tires have a larger diameter, your speedometer will read lower than your actual speed.
Gearing Effect: Larger tires effectively act like a taller gear ratio, potentially reducing acceleration but improving highway cruising RPMs. Smaller tires act like a shorter gear ratio, increasing acceleration but raising highway RPMs.
Always ensure that any new tire size is compatible with your vehicle's specifications and consider the potential impact on your driving experience and safety.
function calculateTireDifference() {
var oldWidth = parseFloat(document.getElementById("oldWidth").value);
var oldAspectRatio = parseFloat(document.getElementById("oldAspectRatio").value);
var oldRimDiameter = parseFloat(document.getElementById("oldRimDiameter").value);
var newWidth = parseFloat(document.getElementById("newWidth").value);
var newAspectRatio = parseFloat(document.getElementById("newAspectRatio").value);
var newRimDiameter = parseFloat(document.getElementById("newRimDiameter").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = "; // Clear previous results
// Input validation
if (isNaN(oldWidth) || isNaN(oldAspectRatio) || isNaN(oldRimDiameter) ||
isNaN(newWidth) || isNaN(newAspectRatio) || isNaN(newRimDiameter) ||
oldWidth <= 0 || oldAspectRatio <= 0 || oldRimDiameter <= 0 ||
newWidth <= 0 || newAspectRatio <= 0 || newRimDiameter <= 0) {
resultDiv.innerHTML = 'Please enter valid positive numbers for all fields.';
return;
}
// Constants
var inchesToMm = 25.4;
var mmToInches = 1 / inchesToMm;
var milesToInches = 63360;
var pi = Math.PI;
// Calculate Old Tire Dimensions
var oldSidewallHeightMM = (oldWidth * oldAspectRatio) / 100;
var oldOverallDiameterMM = (oldSidewallHeightMM * 2) + (oldRimDiameter * inchesToMm);
var oldOverallDiameterInches = oldOverallDiameterMM * mmToInches;
var oldCircumferenceMM = oldOverallDiameterMM * pi;
var oldRevsPerMile = milesToInches / oldOverallDiameterInches;
// Calculate New Tire Dimensions
var newSidewallHeightMM = (newWidth * newAspectRatio) / 100;
var newOverallDiameterMM = (newSidewallHeightMM * 2) + (newRimDiameter * inchesToMm);
var newOverallDiameterInches = newOverallDiameterMM * mmToInches;
var newCircumferenceMM = newOverallDiameterMM * pi;
var newRevsPerMile = milesToInches / newOverallDiameterInches;
// Calculate Differences
var diameterDifferenceMM = newOverallDiameterMM – oldOverallDiameterMM;
var diameterDifferencePercent = (diameterDifferenceMM / oldOverallDiameterMM) * 100;
var circumferenceDifferenceMM = newCircumferenceMM – oldCircumferenceMM;
var circumferenceDifferencePercent = (circumferenceDifferenceMM / oldCircumferenceMM) * 100;
var revsPerMileDifference = newRevsPerMile – oldRevsPerMile;
var revsPerMileDifferencePercent = (revsPerMileDifference / oldRevsPerMile) * 100;
// Display Results
var htmlOutput = 'Comparison Results:';
htmlOutput += 'Old Tire: Overall Diameter = ' + oldOverallDiameterMM.toFixed(2) + ' mm (' + oldOverallDiameterInches.toFixed(2) + ' inches), Circumference = ' + oldCircumferenceMM.toFixed(2) + ' mm, Revs/Mile = ' + oldRevsPerMile.toFixed(2) + ";
htmlOutput += 'New Tire: Overall Diameter = ' + newOverallDiameterMM.toFixed(2) + ' mm (' + newOverallDiameterInches.toFixed(2) + ' inches), Circumference = ' + newCircumferenceMM.toFixed(2) + ' mm, Revs/Mile = ' + newRevsPerMile.toFixed(2) + ";
htmlOutput += '';
htmlOutput += 'Diameter Difference:= 0 ? '#28a745' : '#dc3545') + ';">' + diameterDifferenceMM.toFixed(2) + ' mm (' + diameterDifferencePercent.toFixed(2) + '%)';
htmlOutput += 'Circumference Difference:= 0 ? '#28a745' : '#dc3545') + ';">' + circumferenceDifferenceMM.toFixed(2) + ' mm (' + circumferenceDifferencePercent.toFixed(2) + '%)';
htmlOutput += 'Revolutions Per Mile Difference:= 0 ? '#dc3545' : '#28a745') + ';">' + revsPerMileDifference.toFixed(2) + ' (' + revsPerMileDifferencePercent.toFixed(2) + '%)';
// Speedometer error calculation
var speedometerErrorPercent = diameterDifferencePercent;
if (speedometerErrorPercent > 0) {
htmlOutput += 'Estimated Speedometer Error: Your speedometer will read approximately ' + speedometerErrorPercent.toFixed(2) + '% lower than your actual speed. (e.g., Speedometer shows 60 mph, actual speed is ' + (60 * (1 + speedometerErrorPercent / 100)).toFixed(2) + ' mph)';
} else if (speedometerErrorPercent < 0) {
htmlOutput += 'Estimated Speedometer Error: Your speedometer will read approximately ' + Math.abs(speedometerErrorPercent).toFixed(2) + '% higher than your actual speed. (e.g., Speedometer shows 60 mph, actual speed is ' + (60 * (1 + speedometerErrorPercent / 100)).toFixed(2) + ' mph)';
} else {
htmlOutput += 'Estimated Speedometer Error: No significant speedometer error expected.';
}
resultDiv.innerHTML = htmlOutput;
}