Tyre Size Comparison Calculator
Use this calculator to compare two different tyre sizes and understand how changes in width, aspect ratio, and rim diameter affect your vehicle's speedometer, ground clearance, and overall tyre dimensions.
Original Tyre Specifications
New Tyre Specifications
Tyre Comparison Results
"; resultHTML += "| Metric | Original Tyre | New Tyre | Difference |
|---|---|---|---|
| Tyre Size | " + originalTyreWidth + "/" + originalAspectRatio + "R" + originalRimDiameter + " | " + newTyreWidth + "/" + newAspectRatio + "R" + newRimDiameter + " | |
| Sidewall Height | " + originalSidewallHeight.toFixed(2) + " mm | " + newSidewallHeight.toFixed(2) + " mm | " + (newSidewallHeight – originalSidewallHeight).toFixed(2) + " mm |
| Overall Diameter | " + originalOverallDiameter.toFixed(2) + " mm | " + newOverallDiameter.toFixed(2) + " mm | " + diameterDifference.toFixed(2) + " mm |
| Circumference | " + originalCircumference.toFixed(2) + " mm | " + newCircumference.toFixed(2) + " mm | " + (newCircumference – originalCircumference).toFixed(2) + " mm |
| Revolutions per Km | " + originalRevsPerKm.toFixed(2) + " | " + newRevsPerKm.toFixed(2) + " | " + (newRevsPerKm – originalRevsPerKm).toFixed(2) + " |
| Revolutions per Mile | " + originalRevsPerMile.toFixed(2) + " | " + newRevsPerMile.toFixed(2) + " | " + (newRevsPerMile – originalRevsPerMile).toFixed(2) + " |
| Speedometer Difference | " + speedometerDifferencePercent.toFixed(2) + "% | ||
| Ground Clearance Change | " + groundClearanceChange.toFixed(2) + " mm | ||
| When your speedometer reads " + speedAtOriginal + " km/h (or mph), your actual speed will be approximately | " + speedAtNew.toFixed(2) + " km/h (or mph) | ||
Understanding Tyre Sizes and Their Impact
Changing your vehicle's tyre size is a common modification, whether for aesthetic reasons, performance upgrades, or simply replacing worn-out tyres. However, it's crucial to understand the implications of such changes on your vehicle's performance, safety, and legal compliance. Our Tyre Size Comparison Calculator helps you visualize these impacts.
How to Read a Tyre Size
A typical tyre size, like 205/55R16, breaks down as follows:
- 205: This is the tyre width in millimeters (mm). It measures the width of the tyre from sidewall to sidewall.
- 55: This is the aspect ratio, expressed as a percentage. It represents the height of the tyre's sidewall as a percentage of its width. In this case, the sidewall height is 55% of 205mm.
- R: Indicates a radial construction tyre, which is the most common type today.
- 16: This is the rim diameter in inches. It specifies the diameter of the wheel that the tyre is designed to fit.
Key Metrics Explained
- Sidewall Height: The vertical distance from the wheel rim to the outer tread. A lower aspect ratio means a shorter sidewall, often associated with sportier handling.
- Overall Diameter: The total height of the inflated tyre from the ground to the top. This is critical for speedometer accuracy and fender clearance.
- Circumference: The distance covered by the tyre in one full rotation. Directly related to overall diameter.
- Revolutions per Km/Mile: How many times the tyre rotates to cover one kilometer or mile. A larger diameter tyre will have fewer revolutions per unit distance.
Impact of Changing Tyre Sizes
When you change any of the three main components (width, aspect ratio, or rim diameter), the overall diameter of the tyre can change, leading to several effects:
- Speedometer Accuracy: This is one of the most significant impacts. If the new tyre's overall diameter is different from the original, your speedometer will read incorrectly. A larger diameter tyre will make your speedometer read slower than your actual speed, while a smaller diameter tyre will make it read faster. A difference of more than ±3% is generally not recommended and can lead to speeding tickets or inaccurate mileage tracking.
- Ground Clearance: A larger overall diameter will increase your vehicle's ground clearance, while a smaller one will decrease it. This can affect off-road capability or the vehicle's center of gravity.
- Fender Clearance: Larger tyres might rub against your vehicle's fenders or suspension components, especially during turns or over bumps.
- Handling and Ride Comfort:
- Wider Tyres: Can improve grip and cornering stability but may increase rolling resistance and make steering heavier.
- Lower Aspect Ratio (Shorter Sidewall): Often provides sharper steering response and better handling due to less sidewall flex, but can result in a harsher ride and increased susceptibility to rim damage from potholes.
- Larger Rim Diameter: Typically paired with lower aspect ratio tyres. Can enhance aesthetics and allow for larger brakes, but also contributes to a firmer ride.
- Fuel Economy: Significant changes in tyre size, especially wider tyres or those with different rolling resistance, can affect your vehicle's fuel efficiency.
- Braking Performance: While wider tyres can offer more grip, changes in overall diameter can affect the effective gearing and potentially alter braking dynamics.
Example Scenario:
Let's consider a common change: upgrading from a 205/55R16 to a 225/45R17.
- Original (205/55R16):
- Width: 205 mm
- Aspect Ratio: 55%
- Rim Diameter: 16 inches
- Calculated Overall Diameter: Approximately 631.9 mm
- New (225/45R17):
- Width: 225 mm
- Aspect Ratio: 45%
- Rim Diameter: 17 inches
- Calculated Overall Diameter: Approximately 634.3 mm
In this example, the new tyre is slightly larger in overall diameter (about 2.4 mm). This results in a speedometer difference of roughly +0.38%. This means when your speedometer reads 100 km/h, you're actually going about 100.38 km/h. This is a very minor and acceptable difference.
Always consult your vehicle manufacturer's recommendations or a professional tyre specialist before making significant changes to your tyre size to ensure safety and optimal performance.