Please enter all current and new tire specifications to calculate.
Understanding Rim and Tire Sizes: A Comprehensive Guide
Choosing the right rim and tire combination for your vehicle is crucial for performance, safety, fuel efficiency, and aesthetics. This calculator helps you understand the physical dimensions and potential impact of changing your tire and rim sizes.
How Tire and Rim Sizes Work
Tire sizes are typically expressed using a standardized format, such as P225/55R17. Let's break down what each part means:
P: Indicates a "P-Metric" tire, designed for passenger vehicles. Other prefixes exist for different vehicle types (e.g., LT for Light Truck).
225: Tire Width (in millimeters). This is the measurement from sidewall to sidewall at the widest point.
55: Aspect Ratio (or Sidewall Profile). This is the height of the tire's sidewall as a percentage of its width. In this example, the sidewall height is 55% of 225mm.
R: Indicates a "Radial" construction, which is standard for most modern tires.
17: Rim Diameter (in inches). This is the diameter of the wheel (rim) that the tire is designed to fit.
The Math Behind the Calculator
Our calculator uses these specifications to derive key measurements and compare different tire setups.
Revolutions per Mile = (63,360 inches/mile) / Circumference (inches)
Note: 63,360 is the number of inches in a mile (5280 feet/mile * 12 inches/foot).
Why Does This Matter?
Speedometer Accuracy: A change in overall tire diameter will affect your speedometer and odometer readings. Larger diameter tires (lower revolutions per mile) will make your speedometer read lower than your actual speed. Smaller diameter tires (higher revolutions per mile) will make it read higher.
Gearing and Performance: Changes in tire diameter effectively alter your vehicle's gear ratio. Larger tires can improve fuel economy on the highway but may decrease acceleration. Smaller tires can improve acceleration but may increase engine RPMs at cruising speeds.
Tire Clearance: Ensure that the new tire size fits within your vehicle's wheel wells without rubbing against fenders, suspension components, or other parts, especially during turns or when the suspension is compressed. The calculator helps estimate overall diameter changes, a key factor in clearance.
Handling and Ride Comfort: Aspect ratio plays a significant role. Tires with lower aspect ratios (shorter sidewalls) offer more responsive handling but a firmer ride. Tires with higher aspect ratios provide a more comfortable ride but can feel less precise.
Using the Calculator
Enter your current tire and rim specifications, then enter the desired new tire and rim specifications. The calculator will provide:
The overall diameter of your current and new tires.
The sidewall height of your current and new tires.
The estimated revolutions per mile for both setups.
The difference in overall diameter and revolutions per mile, highlighting potential impacts on your vehicle.
Always consult your vehicle's owner's manual or a professional tire installer to confirm compatible tire and rim sizes for your specific make and model.
function calculateTireFit() {
var tireDiameter = parseFloat(document.getElementById("tireDiameter").value);
var tireWidth = parseFloat(document.getElementById("tireWidth").value);
var aspectRatio = parseFloat(document.getElementById("aspectRatio").value);
var rimDiameter = parseFloat(document.getElementById("rimDiameter").value);
var newTireDiameter = parseFloat(document.getElementById("newTireDiameter").value);
var newTireWidth = parseFloat(document.getElementById("newTireWidth").value);
var newAspectRatio = parseFloat(document.getElementById("newAspectRatio").value);
var newRimDiameter = parseFloat(document.getElementById("newRimDiameter").value);
var resultDisplay = document.getElementById("resultDisplay");
var noResultMsg = document.getElementById("noResult");
if (isNaN(tireDiameter) || isNaN(tireWidth) || isNaN(aspectRatio) || isNaN(rimDiameter) ||
isNaN(newTireDiameter) || isNaN(newTireWidth) || isNaN(newAspectRatio) || isNaN(newRimDiameter)) {
resultDisplay.style.display = 'none';
noResultMsg.style.display = 'block';
return;
}
noResultMsg.style.display = 'none';
resultDisplay.style.display = 'block';
// — Calculations for Current Tire —
var currentSidewallHeight = tireWidth * (aspectRatio / 100);
var currentOverallDiameter = rimDiameter + 2 * (currentSidewallHeight / 25.4);
var currentCircumference = Math.PI * currentOverallDiameter;
var currentRevolutionsPerMile = 63360 / currentCircumference;
// — Calculations for New Tire —
var newSidewallHeight = newTireWidth * (newAspectRatio / 100);
var newOverallDiameter = newRimDiameter + 2 * (newSidewallHeight / 25.4);
var newCircumference = Math.PI * newOverallDiameter;
var newRevolutionsPerMile = 63360 / newCircumference;
// — Differences —
var diameterDifference = newOverallDiameter – currentOverallDiameter;
var revolutionsDifference = newRevolutionsPerMile – currentRevolutionsPerMile;
// — Display Results —
document.getElementById("currentOverallDiameter").innerText = currentOverallDiameter.toFixed(2) + '"';
document.getElementById("currentSidewallHeight").innerText = currentSidewallHeight.toFixed(2) + ' mm';
document.getElementById("currentTireRevolutions").innerText = currentRevolutionsPerMile.toFixed(2);
document.getElementById("newOverallDiameter").innerText = newOverallDiameter.toFixed(2) + '"';
document.getElementById("newSidewallHeight").innerText = newSidewallHeight.toFixed(2) + ' mm';
document.getElementById("newTireRevolutions").innerText = newRevolutionsPerMile.toFixed(2);
document.getElementById("diameterDifference").innerText = diameterDifference.toFixed(2) + '"';
document.getElementById("revolutionsDifference").innerText = revolutionsDifference.toFixed(2);
}
function resetCalculator() {
document.getElementById("tireDiameter").value = "";
document.getElementById("tireWidth").value = "";
document.getElementById("aspectRatio").value = "";
document.getElementById("rimDiameter").value = "";
document.getElementById("newTireDiameter").value = "";
document.getElementById("newTireWidth").value = "";
document.getElementById("newAspectRatio").value = "";
document.getElementById("newRimDiameter").value = "";
document.getElementById("currentOverallDiameter").innerText = "–";
document.getElementById("currentSidewallHeight").innerText = "–";
document.getElementById("currentTireRevolutions").innerText = "–";
document.getElementById("newOverallDiameter").innerText = "–";
document.getElementById("newSidewallHeight").innerText = "–";
document.getElementById("newTireRevolutions").innerText = "–";
document.getElementById("diameterDifference").innerText = "–";
document.getElementById("revolutionsDifference").innerText = "–";
document.getElementById("resultDisplay").style.display = 'none';
document.getElementById("noResult").style.display = 'block';
}