Tire Size Calculator
Compare tire sizes and see how a new size affects your speedometer.
Original Tire Size
Section width in millimeters (e.g., 225)
Sidewall height as a percentage of width (e.g., 45)
Diameter of the wheel in inches (e.g., 17)
New Tire Size
Section width in millimeters (e.g., 245)
Sidewall height as a percentage of width (e.g., 40)
Diameter of the wheel in inches (e.g., 18)
Comparison Results
| Metric | Original Tire | New Tire | Difference |
|---|---|---|---|
| Diameter (in) | |||
| Width (in) | |||
| Sidewall (in) | |||
| Circumference (in) | |||
| Revolutions/Mile |
Diameter Comparison Chart
Speedometer Error
When your speedometer shows 60 mph, your actual speed with the new tires will be 0.0 mph.
Speedometer reading is 0.0% off.
What is a Tire Size Calculator?
A tire size calculator is an essential tool for any car enthusiast or owner looking to change their vehicle’s tires. It helps you understand the physical dimensions of a tire based on the series of numbers found on its sidewall (e.g., 225/45R17). By inputting the size of your current tires and a potential new set, the calculator compares key metrics like overall diameter, section width, sidewall height, and circumference. This comparison is critical for ensuring a new set of tires will fit your vehicle without causing issues like rubbing against the fenders or suspension components. Furthermore, it calculates the potential speedometer error that can occur when you change the overall diameter of your tires.
Tire Size Formula and Explanation
The dimensions of a metric tire are calculated using a specific formula based on its three main numbers: Width, Aspect Ratio, and Rim Diameter. The process involves converting units from millimeters to inches and calculating the sidewall height before determining the final diameter.
- Sidewall Height Calculation: The sidewall height is a percentage of the tire’s width. The formula is:
Sidewall Height (mm) = Width (mm) × (Aspect Ratio / 100) - Overall Diameter Calculation: The total diameter is the sum of the wheel diameter plus two sidewall heights (one on top, one on bottom). Since wheel diameter is in inches, we must convert the sidewall height from millimeters to inches (1 inch = 25.4 mm). The formula is:
Overall Diameter (in) = (2 × Sidewall Height (mm) / 25.4) + Wheel Diameter (in)
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Width | The nominal cross-section width of the tire from sidewall to sidewall. | Millimeters (mm) | 155 – 355 |
| Aspect Ratio | The height of the sidewall as a percentage of the tire’s width. | Percentage (%) | 30 – 80 |
| Wheel Diameter | The diameter of the wheel the tire is designed to be mounted on. | Inches (in) | 14 – 22 |
Practical Examples
Example 1: Upgrading to a Larger Wheel (“Plus-Sizing”)
A common modification is to increase the wheel size for a more aggressive look, a practice known as “plus-sizing”. Let’s see the effect of moving from a standard sedan tire to a larger wheel with a lower-profile tire.
- Original Tire: 215/55R17
- New Tire: 235/40R18
- Analysis: By using the tire size calculator, we’d find the new tire has a similar overall diameter, minimizing speedometer error and ensuring fit. The wider, lower-profile tire on the 18-inch wheel can improve handling and steering response.
Example 2: Increasing Sidewall for Off-Road Use
For off-road vehicles, a larger sidewall is often desired for better cushioning and traction on uneven terrain.
- Original Tire: 265/70R17
- New Tire: 285/70R17
- Analysis: This change keeps the same wheel size but increases width and sidewall height. The tire size calculator would show a significant increase in overall diameter. This would lift the vehicle slightly and provide more sidewall flex. However, it will also cause the speedometer to read slower than the actual speed.
How to Use This Tire Size Calculator
- Enter Original Tire Size: In the “Original Tire Size” section, input the Width, Aspect Ratio, and Wheel Diameter from your current tires. You can find this information on the tire’s sidewall.
- Enter New Tire Size: In the “New Tire Size” section, input the specifications for the tires you are considering.
- Calculate: Click the “Calculate Comparison” button.
- Interpret Results: The calculator will display a detailed table comparing the diameter, width, sidewall, and circumference of both tires. The “Difference” column highlights the changes.
- Check Speedometer Error: The tool shows your actual speed when your speedometer reads 60 mph and quantifies the error percentage. It’s generally recommended to keep the diameter difference within 3% to avoid significant issues.
Key Factors That Affect Tire Size Choices
- Overall Diameter: This is the most crucial factor. A significant change can affect speedometer and odometer accuracy, anti-lock braking systems (ABS), and traction control systems.
- Width: A wider tire can provide a larger contact patch for better grip, but it may also increase fuel consumption and the risk of hydroplaning. Ensure there is enough clearance in the wheel well.
- Sidewall Height: Lower profile (shorter) sidewalls lead to stiffer handling and quicker steering response but a harsher ride. Taller sidewalls offer more comfort and absorb bumps better.
- Wheel Diameter: This is often changed for aesthetic reasons (“plus-sizing”). When you increase wheel diameter, you must decrease the tire’s aspect ratio to maintain a similar overall diameter.
- Load Index: Ensure the new tire has a load index equal to or greater than the original tire to safely support the vehicle’s weight.
- Speed Rating: The new tire should have a speed rating that meets or exceeds the original tire’s rating to maintain safe high-speed performance.
Frequently Asked Questions (FAQ)
Plus-sizing involves increasing the diameter of your wheels while decreasing the tire’s aspect ratio to maintain a similar overall tire diameter. For example, moving from a 16-inch wheel to a 17-inch wheel is called a “plus one” fitment.
Your vehicle’s speedometer is calibrated based on the rotational speed of its original tires. If you install tires with a larger overall diameter, they will cover more ground per revolution, causing your car to travel faster than the speedometer indicates. Conversely, smaller tires will make the speedometer read faster than you are actually going.
It is generally recommended to keep the overall diameter of the new tire within 3% of the original tire’s diameter. Exceeding this can negatively impact gearing, fuel economy, and safety systems like ABS and traction control.
Not always. You must ensure there is enough clearance between the tire and the vehicle’s fender, suspension components, and inner wheel well. A tire size calculator helps compare widths, but a physical check is always recommended.
While not always required just for changing tires of the same size, it is highly recommended, especially if you are changing to a different size. A new set of tires is a significant investment, and proper alignment ensures they wear evenly, maximizing their lifespan.
For a tire marked P225/45R17: ‘P’ is for a passenger car, ‘225’ is the width in millimeters, ’45’ is the aspect ratio (the sidewall is 45% of the width), ‘R’ means radial construction, and ’17’ is the wheel diameter in inches.
Yes, but within limits. Every rim has a recommended range of tire widths it can safely accommodate. Mounting a tire that is too wide or too narrow for the rim can lead to poor handling and unsafe conditions. Check with a tire professional or consult a tire compatibility chart.
Metric sizes (e.g., 285/75R16) use millimeters for width and a percentage for the aspect ratio. Flotation sizes (e.g., 33×12.50R15) are simpler, listing the overall diameter and width directly in inches. Our calculator focuses on the more common metric sizing.