Compare dimensions, circumference, and speedometer error between two tire sizes.
Tire 1 (Current)
Tire 2 (New)
Comparison Results
Metric
Tire 1
Tire 2
Difference
Understanding Tire Size and Comparison
When upgrading wheels or changing tires, understanding the geometric implications is crucial for vehicle safety and performance. A tire size calculator helps you visualize how changing the width, aspect ratio, or rim diameter affects your vehicle's overall height, gearing, and speedometer accuracy.
How to Read Tire Sizes
A standard tire size like 225/50R17 breaks down as follows:
225: The section width of the tire in millimeters.
50: The aspect ratio. This is the sidewall height expressed as a percentage of the width (50% of 225mm).
R17: The diameter of the wheel (rim) in inches that the tire is designed to fit.
Calculations Explained
To find the total diameter of a tire, the formula is:
Total Diameter = ((Width × Aspect Ratio / 100) × 2 / 25.4) + Rim Diameter
We multiply by 2 because there is a sidewall at the top and bottom of the rim. We divide by 25.4 to convert the millimeter measurements into inches to match the rim size.
Impact on Speedometer Accuracy
Your vehicle's speedometer is calibrated based on how many times the tire rotates per mile. If you install a larger diameter tire, the tire travels further with each revolution. This means your speedometer will show a speed slower than you are actually traveling. Conversely, a smaller tire will make your speedometer read faster than your actual speed.
Example Comparison
Suppose you are moving from a 215/65R15 to a 225/70R16:
Tire 1 Diameter: 26.00 inches
Tire 2 Diameter: 28.40 inches
Difference: 9.2% increase
Effect: When your speedometer reads 60 mph, you are actually going 65.5 mph.
Generally, it is recommended to keep the diameter difference within 3% to avoid issues with ABS, traction control, and transmission shift points.
function calculateTireDiff() {
var t1w = parseFloat(document.getElementById('t1w').value);
var t1a = parseFloat(document.getElementById('t1a').value);
var t1r = parseFloat(document.getElementById('t1r').value);
var t2w = parseFloat(document.getElementById('t2w').value);
var t2a = parseFloat(document.getElementById('t2a').value);
var t2r = parseFloat(document.getElementById('t2r').value);
if (!t1w || !t1a || !t1r || !t2w || !t2a || !t2r) {
alert("Please enter all tire specifications.");
return;
}
// Tire 1 Math
var t1SidewallMM = t1w * (t1a / 100);
var t1SidewallInch = t1SidewallMM / 25.4;
var t1Diameter = (t1SidewallInch * 2) + t1r;
var t1Circum = t1Diameter * Math.PI;
var t1Revs = 63360 / t1Circum;
// Tire 2 Math
var t2SidewallMM = t2w * (t2a / 100);
var t2SidewallInch = t2SidewallMM / 25.4;
var t2Diameter = (t2SidewallInch * 2) + t2r;
var t2Circum = t2Diameter * Math.PI;
var t2Revs = 63360 / t2Circum;
// Differences
var sideDiff = ((t2SidewallInch – t1SidewallInch) / t1SidewallInch) * 100;
var diamDiff = ((t2Diameter – t1Diameter) / t1Diameter) * 100;
var circDiff = ((t2Circum – t1Circum) / t1Circum) * 100;
var revsDiff = ((t2Revs – t1Revs) / t1Revs) * 100;
var actualSpeed60 = 60 * (t2Diameter / t1Diameter);
var html = "";
html += "
";
document.getElementById('resultBody').innerHTML = html;
var speedText = "Speedometer Reading: When your speedometer reads 60 mph, you are actually traveling " + actualSpeed60.toFixed(1) + " mph.";
if (Math.abs(diamDiff) > 3) {
speedText += "Warning: Diameter difference is > 3%. This may cause clearance issues or affect vehicle electronics.";
}
document.getElementById('speedoReading').innerHTML = speedText;
document.getElementById('tireResults').style.display = 'block';
}