Tire Size Calculator
Size Comparison
| Measurement | Original Tire | New Tire | Difference |
|---|
Speedometer Error
| Speedo Reads: | Actual Speed: |
|---|
How Does This Work?
This calculator decodes the numbers on your tire's sidewall to provide a detailed comparison between two sizes.
- Reading the Size: For a tire size like 225/45R17, the calculator understands that 225 is the width in millimeters, 45 is the aspect ratio (the sidewall height is 45% of the width), and 17 is the wheel diameter in inches.
- Calculating Sidewall Height: It calculates the height of one sidewall by multiplying the width by the aspect ratio (e.g., 225 mm × 0.45).
- Calculating Overall Diameter: The total diameter of the tire is calculated by taking the wheel diameter and adding the height of *two* sidewalls (one above the wheel, one below). All units are converted to inches for a consistent comparison.
- Comparing Sizes: The calculator performs these calculations for both the original and new tire sizes and then displays the difference in diameter, circumference, and sidewall height. It also calculates the potential speedometer error based on the percentage difference in diameter.
The Surprising History of the Radial Tire
For the first half of the 20th century, all car tires were "bias-ply" tires. They were constructed with overlapping layers of fabric cords running at an angle to the direction of travel. This made them tough but also rigid, leading to a harsh ride, poor handling, and rapid wear.
The breakthrough came from the French tire company Michelin. In 1946, they patented the "radial" tire. In a radial tire, the cords run directly across the tire from bead to bead (radially), with a separate belt of steel or fabric running around the circumference under the tread. This design allowed the sidewall and the tread to function independently. The result was a tire that was far more flexible, provided a smoother ride, had dramatically better grip and handling, and lasted much longer. While it took decades for the radial tire to become the global standard, its invention was the single most important innovation in the history of tire technology.
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Frequently Asked Questions (FAQ)
What is "plus sizing" or "minus sizing"?
This refers to changing your wheel size while keeping the overall tire diameter nearly the same. For example, "plus one" sizing means increasing your wheel diameter by one inch (e.g., from 17" to 18") and choosing a tire with a lower-profile sidewall to compensate. This is done to improve handling and appearance. This calculator is perfect for finding the right tire size for plus or minus sizing.
Will a different tire size fit on my car?
This calculator only tells you the dimensional differences; it cannot determine if a tire will physically fit on your vehicle without rubbing against the fenders, suspension, or brake components. You must always check for proper clearance after installing new tires.
What tire pressure should I use for a different size tire?
The recommended tire pressure is determined by the vehicle manufacturer, not the tire itself. You should generally continue to use the pressure listed on the placard inside your driver's side door jamb, even with a different tire size, unless otherwise specified by a professional tire installer.