For car enthusiasts and everyday drivers alike, changing your vehicle's tire size can have a significant impact on its appearance, performance, and even its speedometer accuracy. Whether you're looking to upgrade to larger wheels for a better look, or choosing a different tire profile for better handling, it's crucial to understand the consequences of that change. Our Tire Size Calculator is an essential tool that allows you to compare a stock tire to a new one, providing a detailed breakdown of the dimensional differences and, most importantly, calculating the effect on your speedometer. This helps you make informed decisions to ensure your new tires are a safe and effective upgrade.
How to Use the Tire Size Calculator
Comparing two tire sizes is a simple process:
- Enter Stock Tire Size: Input the three numbers from your vehicle's original tire, found on its sidewall (e.g., 225 / 45 R 17).
- Enter New Tire Size: Input the three numbers for the new tire you are considering.
- Compare the Sizes: Click the "Compare Sizes" button to see a detailed comparison chart and the resulting speedometer error.
Decoding Your Tire's Sidewall
The numbers on your tire's sidewall aren't random; they provide a precise description of its dimensions. Let's break down a common tire size like 225/45R17.
- 225 (Section Width): This is the width of the tire from sidewall to sidewall, measured in millimeters. In this case, the tire is 225 mm wide.
- 45 (Aspect Ratio): This is the height of the tire's sidewall, expressed as a percentage of its width. Here, the sidewall height is 45% of the 225 mm width (which is 101.25 mm). A lower aspect ratio means a shorter, "lower-profile" sidewall.
- R (Construction Type): This indicates the tire's internal construction. 'R' stands for Radial, which is the standard for virtually all modern passenger car tires.
- 17 (Wheel Diameter): This is the diameter in inches of the wheel that the tire is designed to fit.
Why Does Changing Tire Size Matter?
Changing the overall diameter of your tire and wheel combination can have several important consequences.
Speedometer Error
Your car's speedometer is calibrated from the factory based on the circumference of its original stock tires. It calculates your speed by counting how many times your wheels rotate per second. If you install tires with a larger overall diameter, they will have a larger circumference. This means for each full rotation of the wheel, your car will travel a slightly farther distance than the speedometer expects.
As a result, your actual speed will be faster than your speedometer reading. Conversely, if you install smaller tires, your actual speed will be slower than what your speedometer displays. Our calculator shows you exactly how much this error will be at 60 mph (or km/h), which is crucial for avoiding speeding tickets and ensuring safety. A difference of more than 3% is generally not recommended.
Handling and Ride Comfort
Changing tire size also affects your car's handling.
- "Plus Sizing" (e.g., moving from a 17-inch wheel to an 18-inch wheel) usually involves using a tire with a shorter, stiffer sidewall to keep the overall diameter similar. This can improve steering response and handling precision but often results in a harsher, less comfortable ride, as there is less rubber to absorb bumps in the road.
- Using a tire with a taller sidewall can provide a more cushioned, comfortable ride, but may result in less sharp handling and more body roll in corners.
How to Keep Your Overall Diameter the Same
When upgrading to larger wheels, the goal is usually to choose a new tire size that keeps the overall diameter as close to the original as possible. This minimizes speedometer error and any negative effects on your car's gearing and antilock braking system (ABS). This process is known as "plus sizing."
For example, if you are moving from a 17-inch wheel to an 18-inch wheel ("plus one"), you would typically decrease the tire's aspect ratio to compensate for the larger wheel, keeping the overall diameter nearly identical. Our calculator is the perfect tool for experimenting with different width and aspect ratio combinations to find a new tire that is a good match for your new wheels.
Frequently Asked Questions
How much of a diameter difference is acceptable?
Most tire experts and technicians recommend staying within a 3% difference in overall diameter from the original stock tire size. Anything larger than this can significantly impact your speedometer accuracy, gearing, and potentially interfere with safety systems like ABS and traction control.
Will changing tire size affect my fuel economy?
Yes, it can. Wider tires generally have more rolling resistance, which can slightly decrease your fuel economy. Heavier wheel and tire combinations also require more energy to accelerate, which can negatively impact MPG.
Will new tires fit in my car's wheel well?
This calculator determines the outer dimensions of the tire, but it cannot tell you if it will physically fit on your car without rubbing against the suspension or fenders. If you are making a significant change in width or diameter, it's crucial to consult with a professional tire shop or use an online forum for your specific vehicle model to confirm fitment before purchasing.