TireRack Tire Size Calculator
Compare original equipment (OE) and new tire sizes to understand the differences in dimensions and the effect on your vehicle’s speedometer. This tool helps you make an informed choice, just like the experts at TireRack.
Original Tire Size
The first number in a tire size (e.g., 225/45R17), in millimeters.
The second number (e.g., 225/45R17), the sidewall height as a percentage of the width.
The last number (e.g., 225/45R17), the diameter of the wheel in inches.
New Tire Size
The width of the new tire in millimeters.
The aspect ratio of the new tire.
The diameter of the new wheel in inches.
Visual Comparison
A visual representation of the overall tire diameters.
Speedometer Error
| Speedometer Reads | Actual Speed |
|---|---|
| 25 mph | – |
| 45 mph | – |
| 65 mph | – |
What is a TireRack Tire Size Calculator?
A TireRack tire size calculator is a specialized tool designed to compare the dimensions of two different tire sizes. When considering upgrading or changing your tires, it’s not just about aesthetics; it’s about performance, safety, and compatibility. This calculator helps you understand the critical differences between your vehicle’s original equipment (OE) tire size and a potential new size. By inputting the tire width, aspect ratio, and wheel diameter for both tires, the tool calculates key metrics like overall diameter, sidewall height, and circumference. This allows you to see how a new tire will physically compare and, most importantly, how it will affect your speedometer’s accuracy. Using a reliable tire size calculator is a crucial first step before purchasing new tires, ensuring they fit your vehicle without issues and that you’re aware of any potential performance changes.
Tire Size Formula and Explanation
Understanding the numbers on a tire’s sidewall is key to calculating its size. The standard format (e.g., 225/45R17) mixes millimeters, percentages, and inches. The formula to find the total diameter of a tire is:
Overall Diameter (in) = 2 × (Width × Aspect Ratio / 100 / 25.4) + Wheel Diameter
This formula first calculates the height of one sidewall. Since the overall diameter includes the wheel plus the sidewall on both the top and bottom, we must multiply the single sidewall height by two. Because the tire width is in millimeters and the wheel diameter is in inches, a conversion (25.4mm = 1 inch) is necessary for an accurate result.
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Width | The nominal 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 – 85 |
| Wheel Diameter | The diameter of the wheel the tire is designed to fit on. | Inches (in) | 14 – 24 |
Practical Examples
Example 1: A Common “Plus One” Upgrade
A common modification is to increase the wheel size by one inch while adjusting the tire size to maintain a similar overall diameter. This is often done for aesthetic and handling improvements.
- Original Tire: 215/55R17
- New Tire: 225/45R18
- Analysis: The original tire has a diameter of 26.3 inches. The new “plus one” tire has a diameter of 25.9 inches. This is a -1.5% difference, which is well within the recommended 3% variance. The speedometer will read slightly faster than the actual speed. For a deeper analysis, you can check a tire tech resource.
Example 2: Off-Road Truck Tire Upgrade
Truck owners often want larger, more aggressive tires for off-road capability and appearance.
- Original Tire: 265/70R17 (approx. 31.6″ diameter)
- New Tire: 285/70R17 (approx. 32.7″ diameter)
- Analysis: The new tire is 1.1 inches taller, a +3.5% difference. This is slightly outside the general 3% guideline and will have a noticeable effect. Your speedometer will read slower than your actual speed (e.g., at an indicated 60 mph, your actual speed would be ~62.1 mph). This also impacts gearing and may require speedometer recalibration.
How to Use This TireRack Tire Size Calculator
- Enter Original Tire Size: In the left column, input the three numbers from your current tire’s sidewall (e.g., 225, 45, 17).
- Enter New Tire Size: In the right column, input the size of the tires you are considering.
- Review the Results: The calculator instantly updates. The “Diameter Difference” is the most important number; experts recommend keeping this within +/- 3% of the original size to avoid issues with braking, gearing, and electronic safety systems.
- Check Speedometer Error: The table shows how the new tire size will affect your speed. A larger tire will make you travel faster than the speedometer indicates, while a smaller tire will do the opposite.
- Visualize the Change: The bar chart gives you a quick visual reference for the change in overall tire height. For more details on tire data, our guide on how to calculate tire dimensions provides in-depth info.
Key Factors That Affect Tire Sizing
- Speedometer and Odometer Accuracy: As demonstrated by the calculator, changing the tire’s overall diameter alters its circumference, which directly impacts speed and mileage readings.
- Fitment and Clearance: A significantly larger tire may rub against the suspension, fender liners, or bodywork, especially during turns or suspension compression.
- Ride Comfort: The aspect ratio determines the sidewall height. A shorter sidewall (lower aspect ratio) generally leads to a firmer ride and quicker handling response, while a taller sidewall provides more cushioning.
- Gearing and Acceleration: A taller tire effectively lengthens your vehicle’s final drive ratio, which can slightly reduce acceleration but may lower engine RPM at highway speeds.
- Load Index and Speed Rating: Any new tire must have a load index and speed rating that meets or exceeds the vehicle manufacturer’s specifications to ensure safety. Check out our tech center article on speed rating.
- Braking System: Anti-lock Braking Systems (ABS) and Electronic Stability Control (ESC) are calibrated for the original tire diameter. A significant change can potentially affect their performance. It’s important to find the right tires that fit your vehicle.
Frequently Asked Questions (FAQ)
- What is the 3% rule for changing tire size?
- The 3% rule is a general guideline suggesting that your new tires should have an overall diameter that is within 3% (either larger or smaller) of your original tires. This helps minimize the impact on your vehicle’s speedometer, ABS, and traction control systems.
- Will bigger tires affect my fuel economy?
- Yes, they can. Larger, heavier tires increase rolling resistance and unsprung weight, which typically leads to a decrease in fuel efficiency. The change in effective gearing can also play a role.
- If my speedometer is off, how do I fix it?
- For many modern vehicles, a dealership or a specialized shop can recalibrate the engine control unit (ECU) for the new tire size. For some vehicles, aftermarket programmers are available that allow you to make this change yourself.
- What is “plus sizing”?
- Plus sizing refers to increasing the diameter of the wheel (“plus one,” “plus two,” etc.) 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 a “plus one” upgrade.
- Do I need a new alignment after getting new tires?
- 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. It ensures your new tires wear evenly and the vehicle handles correctly. For professional advice, consult a resource like a tire size expert.
- Can I put wider tires on my stock rims?
- Each rim has a recommended range of tire widths it can safely accommodate. Installing a tire that is too wide for the rim can lead to poor handling, uneven wear, and an increased risk of the tire debeading from the rim.
- How does sidewall height affect performance?
- A shorter sidewall provides less flex, leading to sharper, more responsive handling but a firmer ride. A taller sidewall offers more cushioning for a smoother ride but can feel less precise during cornering.
- Is a bigger tire always better?
- Not necessarily. While a larger tire might look better or offer more off-road grip, it can negatively impact fuel economy, acceleration, and ride comfort. The best tire size is one that fits your vehicle correctly and matches your driving needs, a concept you can explore with a tire markings guide.