Compare two tire sizes to see how they affect your vehicle's diameter, speedometer accuracy, and ride height.
Tire 1 (Stock)
Tire 2 (New)
Comparison Results
Metric
Tire 1
Tire 2
Difference
Understanding Tire Size Math
When you look at a tire sidewall, you see a code like 225/45R17. These numbers tell a specific story about the tire's physical dimensions. Changing these numbers can significantly impact your car's performance, fuel economy, and speedometer accuracy.
1. Section Width (mm)
The first number (225) is the width of the tire in millimeters from sidewall to sidewall. A wider tire generally provides more grip but can increase rolling resistance.
2. Aspect Ratio (%)
The second number (45) is the aspect ratio. It represents the height of the sidewall as a percentage of the width. In this case, the sidewall height is 45% of 225mm.
3. Wheel Diameter (inches)
The final number (17) is the diameter of the wheel (rim) the tire is designed to fit, measured in inches.
The 3% Safety Rule
Most tire experts and mechanics recommend staying within 3% of the original tire's total diameter. If your new tires are more than 3% larger or smaller than the factory specifications, you risk:
Brake failure due to increased leverage
Inaccurate speedometer and odometer readings
Transmission shift point issues (in automatics)
Rubbing against the fender or suspension components
In this case, when your speedometer reads 60 mph, you would actually be traveling roughly 57.6 mph.
function calculateTireDifference() {
var w1 = parseFloat(document.getElementById('width1').value);
var a1 = parseFloat(document.getElementById('aspect1').value);
var d1 = parseFloat(document.getElementById('wheel1').value);
var w2 = parseFloat(document.getElementById('width2').value);
var a2 = parseFloat(document.getElementById('aspect2').value);
var d2 = parseFloat(document.getElementById('wheel2').value);
if (isNaN(w1) || isNaN(a1) || isNaN(d1) || isNaN(w2) || isNaN(a2) || isNaN(d2)) {
alert("Please enter valid numbers for all fields.");
return;
}
// Calculations for Tire 1
var sidewall1 = w1 * (a1 / 100);
var totalDia1 = (sidewall1 * 2) + (d1 * 25.4); // Total diameter in mm
var circum1 = totalDia1 * Math.PI;
var revsPerKm1 = 1000000 / circum1;
// Calculations for Tire 2
var sidewall2 = w2 * (a2 / 100);
var totalDia2 = (sidewall2 * 2) + (d2 * 25.4); // Total diameter in mm
var circum2 = totalDia2 * Math.PI;
var revsPerKm2 = 1000000 / circum2;
// Differences
var diaDiffMm = totalDia2 – totalDia1;
var diaDiffPct = (diaDiffMm / totalDia1) * 100;
// Ride height difference (half of diameter difference)
var rideHeightDiff = diaDiffMm / 2;
var resultBody = document.getElementById('resultBody');
var html = "";
// Diameter Row
html += "