Tire Sizing Calculator
Original Tire Size
(e.g., for 225/45R17)
New Tire Size
(e.g., for 245/40R18)
Calculation Results
Enter tire dimensions and click 'Calculate' to see the comparison.
Error:
Please enter valid positive numbers for all tire dimensions.'; return; } // Helper function to calculate tire dimensions function getTireDimensions(width_mm, aspect_ratio_percent, wheel_diameter_inches) { var sidewall_mm = width_mm * (aspect_ratio_percent / 100); var sidewall_inches = sidewall_mm / 25.4; var overall_diameter_inches = wheel_diameter_inches + (2 * sidewall_inches); var overall_diameter_mm = overall_diameter_inches * 25.4; var circumference_inches = overall_diameter_inches * Math.PI; var revs_per_mile = 63360 / circumference_inches; // 1 mile = 63360 inches return { sidewall_mm: sidewall_mm, sidewall_inches: sidewall_inches, overall_diameter_inches: overall_diameter_inches, overall_diameter_mm: overall_diameter_mm, circumference_inches: circumference_inches, revs_per_mile: revs_per_mile }; } // Calculate dimensions for original tire var originalDims = getTireDimensions(originalTireWidth, originalAspectRatio, originalWheelDiameter); // Calculate dimensions for new tire var newDims = getTireDimensions(newTireWidth, newAspectRatio, newWheelDiameter); // Calculate differences var diff_diameter_inches = newDims.overall_diameter_inches – originalDims.overall_diameter_inches; var diff_diameter_mm = newDims.overall_diameter_mm – originalDims.overall_diameter_mm; var diff_diameter_percent = (diff_diameter_inches / originalDims.overall_diameter_inches) * 100; var speedometer_error_percent = diff_diameter_percent; // If new tire is larger, speedometer reads lower (under-reads). If smaller, over-reads. var ground_clearance_change_inches = diff_diameter_inches / 2; var ground_clearance_change_mm = diff_diameter_mm / 2; var revs_per_mile_diff = newDims.revs_per_mile – originalDims.revs_per_mile; // Format results var resultsHtml = 'Tire Comparison Results
'; resultsHtml += '';
// Original Tire Details
resultsHtml += '
'; // Close flex container
// Differences Summary
resultsHtml += '';
resultsHtml += '
';
// New Tire Details
resultsHtml += 'Original Tire (' + originalTireWidth + '/' + originalAspectRatio + 'R' + originalWheelDiameter + ')
'; resultsHtml += 'Sidewall Height: ' + originalDims.sidewall_inches.toFixed(2) + ' inches (' + originalDims.sidewall_mm.toFixed(1) + ' mm)'; resultsHtml += 'Overall Diameter: ' + originalDims.overall_diameter_inches.toFixed(2) + ' inches (' + originalDims.overall_diameter_mm.toFixed(1) + ' mm)'; resultsHtml += 'Circumference: ' + originalDims.circumference_inches.toFixed(2) + ' inches'; resultsHtml += 'Revolutions per Mile: ' + originalDims.revs_per_mile.toFixed(2) + "; resultsHtml += '';
resultsHtml += '
';
resultsHtml += 'New Tire (' + newTireWidth + '/' + newAspectRatio + 'R' + newWheelDiameter + ')
'; resultsHtml += 'Sidewall Height: ' + newDims.sidewall_inches.toFixed(2) + ' inches (' + newDims.sidewall_mm.toFixed(1) + ' mm)'; resultsHtml += 'Overall Diameter: ' + newDims.overall_diameter_inches.toFixed(2) + ' inches (' + newDims.overall_diameter_mm.toFixed(1) + ' mm)'; resultsHtml += 'Circumference: ' + newDims.circumference_inches.toFixed(2) + ' inches'; resultsHtml += 'Revolutions per Mile: ' + newDims.revs_per_mile.toFixed(2) + "; resultsHtml += '';
resultsHtml += '
';
resultDiv.innerHTML = resultsHtml;
}
Key Differences
'; resultsHtml += 'Overall Diameter Difference: ' + (diff_diameter_inches > 0 ? '+' : ") + diff_diameter_inches.toFixed(2) + ' inches (' + (diff_diameter_mm > 0 ? '+' : ") + diff_diameter_mm.toFixed(1) + ' mm)'; resultsHtml += 'Diameter Change Percentage: ' + (diff_diameter_percent > 0 ? '+' : ") + speedometer_error_percent.toFixed(2) + '%'; resultsHtml += 'Speedometer Error: ' + (speedometer_error_percent > 0 ? 'Your speedometer will read ' + Math.abs(speedometer_error_percent).toFixed(2) + '% SLOWER than actual speed.' : speedometer_error_percent < 0 ? 'Your speedometer will read ' + Math.abs(speedometer_error_percent).toFixed(2) + '% FASTER than actual speed.' : 'No significant speedometer error.') + ''; if (speedometer_error_percent !== 0) { resultsHtml += '(If your speedometer reads 60 MPH, your actual speed will be approximately ' + (60 * (1 + speedometer_error_percent / 100)).toFixed(1) + ' MPH)'; } resultsHtml += 'Ground Clearance Change: ' + (ground_clearance_change_inches > 0 ? '+' : ") + ground_clearance_change_inches.toFixed(2) + ' inches (' + (ground_clearance_change_mm > 0 ? '+' : ") + ground_clearance_change_mm.toFixed(1) + ' mm)'; resultsHtml += 'Revolutions per Mile Change: ' + (revs_per_mile_diff > 0 ? '+' : ") + revs_per_mile_diff.toFixed(2) + "; resultsHtml += 'Understanding Tire Sizing and Its Impact
Changing your vehicle's tire size is a common modification, whether for aesthetic reasons, performance upgrades, or adapting to different driving conditions like off-roading. However, it's crucial to understand the implications of such changes on your vehicle's performance, safety, and accuracy of its systems.
How to Read Tire Sizes
Tire sizes are typically expressed in a format like 225/45R17:
- 225: This is the Tire Width in millimeters. It measures the width of the tire from sidewall to sidewall.
- 45: This is the Aspect Ratio, expressed as a percentage. It represents the sidewall height as a percentage of the tire's width. In this example, the sidewall height is 45% of 225mm.
- R: Indicates a radial construction tire, which is standard for most modern vehicles.
- 17: This is the Wheel Diameter in inches. It specifies the diameter of the wheel that the tire is designed to fit.
Key Calculations Explained
Our calculator takes your original and new tire dimensions to provide a detailed comparison:
- Sidewall Height: The vertical distance from the wheel rim to the outer tread. A taller sidewall generally provides a softer ride, while a shorter sidewall offers better handling and responsiveness.
- Overall Diameter: The total height of the tire from the ground to the top. This is a critical measurement as it directly affects speedometer accuracy and ground clearance.
- Circumference: The distance covered by one full rotation of the tire. This is directly related to the overall diameter.
- Revolutions per Mile: The number of times the tire rotates to travel one mile. A larger tire will have fewer revolutions per mile than a smaller one.
Impacts of Changing Tire Size
Modifying your tire size can have several significant effects:
- Speedometer and Odometer Error: This is one of the most common and important impacts. If your new tires have a larger overall diameter, your speedometer will read slower than your actual speed, and your odometer will record fewer miles than you've truly traveled. Conversely, smaller tires will cause your speedometer to read faster and your odometer to record more miles. This can lead to speeding tickets or inaccurate service intervals.
- Ground Clearance: A larger overall diameter will increase your vehicle's ground clearance, which can be beneficial for off-roading. A smaller diameter will reduce it.
- Gearing and Performance: Changing tire diameter effectively alters your vehicle's final drive ratio. Larger tires make your effective gearing "taller," meaning the engine has to work harder to achieve the same speed, potentially reducing acceleration and fuel economy. Smaller tires make gearing "shorter," improving acceleration but possibly reducing top speed and increasing engine RPM at highway speeds.
- Fuel Economy: Larger, heavier tires can increase rolling resistance and unsprung weight, leading to a decrease in fuel efficiency.
- Handling and Ride Quality: Changes in sidewall height and tire width can significantly alter how your vehicle handles corners, absorbs bumps, and feels on the road.
- Fender and Suspension Clearance: Larger tires might rub against your vehicle's fenders, suspension components, or inner wheel wells, especially during turns or when the suspension compresses.
- Braking Performance: Larger or heavier tires can increase the rotational inertia, potentially affecting braking distances.
Using the Calculator
To use this calculator, simply input the width, aspect ratio, and wheel diameter for both your original tires and the new tires you are considering. The calculator will then provide a detailed breakdown of each tire's dimensions and highlight the key differences, including speedometer error and ground clearance changes. Always consult with a professional tire specialist before making significant changes to your vehicle's tire setup.
Example Scenario:
Let's say your original tires are 225/45R17, and you're considering upgrading to 245/40R18.
- Original Tire (225/45R17):
- Sidewall Height: ~3.99 inches (101.3 mm)
- Overall Diameter: ~24.97 inches (634.3 mm)
- Revolutions per Mile: ~807.7
- New Tire (245/40R18):
- Sidewall Height: ~3.86 inches (98.0 mm)
- Overall Diameter: ~25.72 inches (653.2 mm)
- Revolutions per Mile: ~784.3
- Key Differences:
- Overall Diameter Difference: +0.75 inches (+18.9 mm)
- Diameter Change Percentage: +3.00%
- Speedometer Error: Your speedometer will read approximately 3.00% SLOWER than actual speed. If it reads 60 MPH, you're actually going ~61.8 MPH.
- Ground Clearance Change: +0.37 inches (+9.5 mm)
This example shows that the new tires are slightly larger, which would increase ground clearance but also cause your speedometer to under-read. This information is vital for making an informed decision.