Calculate Tire

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Tire Size Comparison Calculator

Compare two tire sizes to see how they affect your vehicle's height, speedometer, and performance.

Tire 1 (Original)

Example: 225
Example: 45
Example: 17

Tire 2 (New)

Example: 245
Example: 40
Example: 18

Comparison Results

Metric Tire 1 Tire 2 Difference
function calculateTireComparison() { var w1 = parseFloat(document.getElementById('t1_w').value); var ar1 = parseFloat(document.getElementById('t1_ar').value); var rd1 = parseFloat(document.getElementById('t1_rd').value); var w2 = parseFloat(document.getElementById('t2_w').value); var ar2 = parseFloat(document.getElementById('t2_ar').value); var rd2 = parseFloat(document.getElementById('t2_rd').value); if (isNaN(w1) || isNaN(ar1) || isNaN(rd1) || isNaN(w2) || isNaN(ar2) || isNaN(rd2)) { alert("Please fill in all fields with valid numbers."); return; } // Calculations for Tire 1 var sw1_mm = w1 * (ar1 / 100); var sw1_in = sw1_mm / 25.4; var d1 = (sw1_in * 2) + rd1; var circ1 = d1 * Math.PI; var rpm1 = 63360 / circ1; // Calculations for Tire 2 var sw2_mm = w2 * (ar2 / 100); var sw2_in = sw2_mm / 25.4; var d2 = (sw2_in * 2) + rd2; var circ2 = d2 * Math.PI; var rpm2 = 63360 / circ2; var d_diff = ((d2 – d1) / d1) * 100; var speed_error = (d2 / d1); var actual_60 = 60 * speed_error; var resultBody = document.getElementById('result-body'); resultBody.innerHTML = 'Total Diameter' + '' + d1.toFixed(2) + '"' + '' + d2.toFixed(2) + '"' + '' + d_diff.toFixed(1) + '%' + 'Sidewall Height' + '' + sw1_in.toFixed(2) + '"' + '' + sw2_in.toFixed(2) + '"' + '' + (sw2_in – sw1_in).toFixed(2) + '"' + 'Circumference' + '' + circ1.toFixed(2) + '"' + '' + circ2.toFixed(2) + '"' + '' + (circ2 – circ1).toFixed(2) + '"' + 'Revs per Mile' + '' + rpm1.toFixed(0) + '' + '' + rpm2.toFixed(0) + '' + '' + (rpm2 – rpm1).toFixed(0) + ''; var speedoBox = document.getElementById('speedo-box'); var errorColor = Math.abs(d_diff) > 3 ? "#e74c3c" : "#27ae60"; var errorLight = Math.abs(d_diff) > 3 ? "#fdecea" : "#eafaf1"; speedoBox.style.backgroundColor = errorLight; speedoBox.style.color = errorColor; speedoBox.innerHTML = "When your speedometer reads 60 MPH, you are actually traveling " + actual_60.toFixed(1) + " MPH. Total diameter difference is " + d_diff.toFixed(2) + "%."; document.getElementById('tire-results').style.display = 'block'; }

Understanding Tire Sizes and Their Impact

Changing your tire or wheel size is a popular modification, but it affects more than just the look of your vehicle. The total diameter of the tire determines how far your car travels with every revolution of the axle. When you change this diameter, you change your gearing, speedometer accuracy, and ride height.

How to Read Tire Measurements

If you look at the sidewall of your tire, you will see a string of numbers like 225/45R17. Here is what they mean:

  • Width (225): The width of the tire from sidewall to sidewall in millimeters.
  • Aspect Ratio (45): This is the height of the sidewall expressed as a percentage of the width. In this case, the sidewall is 45% of 225mm.
  • Wheel Diameter (17): The diameter of the wheel (rim) in inches.

Speedometer Error Explained

Your vehicle's speedometer is calibrated based on the factory tire diameter. If you install a larger tire, the tire covers more ground per revolution, meaning you are actually going faster than the speedometer indicates. Conversely, a smaller tire will result in a speedometer reading that is higher than your actual speed.

The 3% Rule

Most automotive experts recommend keeping the total diameter change within 3% of the original factory size. Exceeding this limit can lead to several issues:

  1. Braking Performance: Larger tires increase the leverage against your brakes, potentially increasing stopping distances.
  2. Transmission Stress: Significant changes in diameter can confuse automatic transmission shift points.
  3. Clearance Issues: Oversized tires may rub against the fender liners or suspension components during sharp turns or bumps.
  4. ABS and Traction Control: Modern safety systems rely on wheel speed sensors; drastic changes can cause error codes or improper system activation.

Example Comparison

Consider upgrading from a standard 205/55R16 to a 225/40R18:

  • 205/55R16: Total Diameter = 24.88 inches.
  • 225/40R18: Total Diameter = 25.09 inches.
  • Result: A 0.8% increase. This is a very safe upgrade as the speedometer error is negligible (only 0.5 MPH difference at highway speeds) and the diameter remains almost identical.

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