Tire Diameter Calculator

Tire Diameter Calculator

Calculated Dimensions

Total Diameter (inches): 0"

Total Diameter (mm): 0 mm

Sidewall Height: 0"

Circumference: 0"

Revolutions per Mile: 0

function calculateTireDiameter() { var width = parseFloat(document.getElementById('tireWidth').value); var aspect = parseFloat(document.getElementById('aspectRatio').value); var rim = parseFloat(document.getElementById('rimSize').value); if (isNaN(width) || isNaN(aspect) || isNaN(rim) || width <= 0 || aspect <= 0 || rim <= 0) { alert("Please enter valid positive numbers for all fields."); return; } // Calculation Logic // Sidewall height = Width * (Aspect Ratio / 100) var sidewallMm = width * (aspect / 100); var sidewallIn = sidewallMm / 25.4; // Total diameter = (Sidewall * 2) + Rim Diameter var totalDiameterIn = (sidewallIn * 2) + rim; var totalDiameterMm = totalDiameterIn * 25.4; // Circumference = Diameter * PI var circumferenceIn = totalDiameterIn * Math.PI; // Revolutions per mile = 63360 (inches in a mile) / circumference var revPerMile = 63360 / circumferenceIn; // Display Results document.getElementById('resInches').innerText = totalDiameterIn.toFixed(2); document.getElementById('resMm').innerText = totalDiameterMm.toFixed(1); document.getElementById('resSidewall').innerText = sidewallIn.toFixed(2); document.getElementById('resCirc').innerText = circumferenceIn.toFixed(2); document.getElementById('resRev').innerText = Math.round(revPerMile); document.getElementById('tireResults').style.display = 'block'; }

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When upgrading your wheels or changing your tire size, understanding the total tire diameter is crucial. The diameter affects everything from your speedometer accuracy to your vehicle's gear ratios and wheel well clearance. This calculator helps you convert standard metric tire sizes into physical dimensions like inches and millimeters.

How to Read a Tire Size

Most modern tires use a standardized metric format, such as 245/40 R18. Each number provides a specific measurement:

• 245 (Width): This is the sectional width of the tire in millimeters from sidewall to sidewall.
• 40 (Aspect Ratio): This is the height of the sidewall expressed as a percentage of the width. In this case, the sidewall is 40% of 245mm.
• 18 (Wheel Diameter): This is the diameter of the wheel (rim) in inches that the tire is designed to fit.

The Math Behind the Calculation

To find the total height of a tire, we use the following formula:

Total Diameter = ((Width × Aspect Ratio / 100) × 2 / 25.4) + Rim Diameter

Because there is a sidewall on the top and the bottom of the wheel, we must multiply the sidewall height by two before adding the wheel diameter. Since width is in millimeters and the rim is in inches, we divide by 25.4 to keep units consistent.

Why Diameter Accuracy Matters

Vehicles are engineered with a specific rolling diameter in mind. If you install tires that are significantly taller or shorter than the factory equipment, you may experience:

• Speedometer Error: A larger diameter means your actual speed is higher than what the speedometer shows.
• Odometer Inaccuracy: Your car will record fewer miles than you have actually traveled if the tires are too large.
• Transmission Issues: On modern automatic cars, significant changes in tire diameter can affect shift points.
• Clearance Problems: Oversized tires may rub against the suspension components or the fender liner when turning or hitting bumps.

Example Calculation: 275/70 R18 (Truck Tire)

Let's look at a common heavy-duty truck tire size: 275/70 R18.

1. Sidewall Height: 275mm × 0.70 = 192.5 mm.
2. Convert to Inches: 192.5 mm / 25.4 = 7.58 inches.
3. Double for Top/Bottom: 7.58″ × 2 = 15.16 inches.
4. Add Rim Size: 15.16″ + 18″ = 33.16 inches total diameter.

This tire would commonly be referred to as a "33-inch tire" in the off-road community.