Tyre Size Comparison Calculator
Use this calculator to compare two different tire sizes and understand how changing your tire dimensions can affect your vehicle's performance, speedometer accuracy, and ground clearance.
Original Tyre Size
New Tyre Size
Understanding Tyre Size Comparison
Changing your vehicle's tyre size is a common modification, whether for aesthetic reasons, performance upgrades, or simply replacing worn-out tyres with a different specification. However, altering tyre dimensions can have significant impacts on your vehicle's handling, fuel economy, and even the accuracy of your speedometer. Our Tyre Size Comparison Calculator helps you understand these changes before you make a purchase.
How Tyre Sizes Are Designated
Tyre sizes are typically displayed as a series of numbers and letters, for example, 205/55R16. Let's break down what each part means:
- 205 (Tire Width): This is the width of the tyre in millimeters, measured from sidewall to sidewall. A wider tyre generally offers more grip but can increase rolling resistance.
- 55 (Aspect Ratio): This number represents the sidewall height as a percentage of the tyre's width. In this example, the sidewall height is 55% of 205mm. A lower aspect ratio means a shorter sidewall, often found on performance tyres, offering better handling but a harsher ride.
- R (Construction Type): 'R' stands for Radial, which is the most common type of tyre construction today.
- 16 (Wheel Diameter): This is the diameter of the wheel (or rim) in inches that the tyre is designed to fit.
Why Compare Tyre Sizes?
Before switching to a different tyre size, it's crucial to understand the implications:
- Speedometer Accuracy: If the overall diameter of your new tyres is different from your original ones, your speedometer will no longer read accurately. A larger diameter tyre will make your speedometer read lower than your actual speed, while a smaller diameter tyre will make it read higher. This can lead to speeding tickets or misjudging distances.
- Ground Clearance: A larger overall diameter will increase your vehicle's ground clearance, which can be beneficial for off-roading but might raise the center of gravity. A smaller diameter will reduce ground clearance.
- Handling and Ride Comfort: Changes in sidewall height directly impact ride comfort and handling. Shorter sidewalls (lower aspect ratio) generally provide sharper steering response and better cornering but can result in a firmer, less comfortable ride. Taller sidewalls (higher aspect ratio) offer more cushioning and a smoother ride but might feel less responsive.
- Fuel Economy: Larger or wider tyres can increase rolling resistance and overall weight, potentially leading to a decrease in fuel efficiency.
- Fitment Issues: Tyres that are too wide or have too large a diameter might rub against suspension components, fender liners, or bodywork, especially during turns or when the suspension compresses.
- Braking Performance: While wider tires can offer more grip, a significant change in overall diameter can affect the effective gearing and potentially alter braking dynamics.
How the Calculator Works
Our calculator takes the width, aspect ratio, and wheel diameter for both your original and new tyres. It then calculates and compares several key metrics:
- Sidewall Height: The actual height of the tyre's sidewall.
- Overall Diameter: The total height of the mounted tyre from top to bottom.
- Circumference: The distance covered by one full rotation of the tyre.
- Revolutions per Mile/Kilometer: How many times the tyre rotates to cover a mile or kilometer. This is directly related to speedometer accuracy.
- Speedometer Error: The percentage difference in speed reading and an estimate of your actual speed if your speedometer reads 60 mph/kmh.
- Ground Clearance Change: How much your vehicle's lowest point will be raised or lowered.
Example Scenario:
Let's say your original tyres are 205/55R16 and you're considering upgrading to 225/45R17. Here's what the calculator would reveal:
- Original Tyre (205/55R16):
- Sidewall Height: 112.75 mm
- Overall Diameter: 631.9 mm (24.88 inches)
- Circumference: 1985.2 mm (78.16 inches)
- Revolutions per Mile: ~810
- New Tyre (225/45R17):
- Sidewall Height: 101.25 mm
- Overall Diameter: 634.3 mm (24.97 inches)
- Circumference: 1992.8 mm (78.46 inches)
- Revolutions per Mile: ~807
Comparison:
- Diameter Difference: The new tyre is slightly larger by about 2.4 mm (0.09 inches).
- Speedometer Reading: If your speedometer reads 60 mph, you would actually be going approximately 60.23 mph. Your speedometer will read 0.38% lower than your actual speed.
- Ground Clearance Change: Your ground clearance would increase by about 1.2 mm (0.05 inches).
- Sidewall Height: The new tyre has a shorter sidewall by 11.5 mm, which could lead to a firmer ride and potentially better handling.
This example demonstrates how even seemingly small changes in tyre dimensions can impact critical vehicle characteristics. Always consult your vehicle manufacturer's recommendations and a professional tyre specialist before making significant changes to your tyre size.
Comparison Results
"; resultsHTML += "| Metric | Original Tyre | New Tyre | Difference (New – Original) |
|---|---|---|---|
| Tire Size | " + originalWidth + "/" + originalAspect + "R" + originalWheel + " | " + newWidth + "/" + newAspect + "R" + newWheel + " | |
| Sidewall Height | " + originalSidewallHeightMM.toFixed(2) + " mm | " + newSidewallHeightMM.toFixed(2) + " mm | " + sidewallDifferenceMM.toFixed(2) + " mm |
| Overall Diameter | " + originalOverallDiameterMM.toFixed(2) + " mm (" + originalOverallDiameterInches.toFixed(2) + " in) | " + newOverallDiameterMM.toFixed(2) + " mm (" + newOverallDiameterInches.toFixed(2) + " in) | " + diameterDifferenceMM.toFixed(2) + " mm (" + diameterDifferenceInches.toFixed(2) + " in) |
| Circumference | " + originalCircumferenceMM.toFixed(2) + " mm (" + originalCircumferenceInches.toFixed(2) + " in) | " + newCircumferenceMM.toFixed(2) + " mm (" + newCircumferenceInches.toFixed(2) + " in) | " + circumferenceDifferenceMM.toFixed(2) + " mm (" + circumferenceDifferenceInches.toFixed(2) + " in) |
| Revolutions per Mile | " + originalRevsPerMile.toFixed(2) + " | " + newRevsPerMile.toFixed(2) + " | " + revsPerMileDifference.toFixed(2) + " |
| Revolutions per Kilometer | " + originalRevsPerKM.toFixed(2) + " | " + newRevsPerKM.toFixed(2) + " | " + revsPerKMDifference.toFixed(2) + " |