Tire Size Calculator

A tire size calculator is a tool that compares two tire sizes and instantly reveals the practical differences between them — overall diameter, sidewall height, circumference, revolutions per mile, ground clearance, and speedometer accuracy. Whether you're upgrading your wheels, switching to a different tire, or just curious how a size change will affect your vehicle, this calculator turns confusing tire codes into clear, actionable numbers.

Every tire has a size stamped on its sidewall in a format like 225/45R17. These three numbers describe the tire's width, profile, and wheel fitment, but they don't directly tell you how one tire compares to another. Two tires with completely different codes can have nearly identical rolling diameters, while two similar-looking sizes can differ enough to throw off your speedometer or cause rubbing. This calculator does the geometry for you.

The most important output is the overall diameter difference. Tire and vehicle manufacturers recommend keeping a replacement tire within 3% of the original diameter. Staying inside this window preserves speedometer accuracy, protects your anti-lock braking and traction control systems, and avoids clearance problems. This calculator highlights whether your comparison is within that safe tolerance.

A tire size like 225/45R17 encodes everything needed to compute its real-world dimensions:

Worked example: A 225/45R17 tire has a sidewall of 225 × 0.45 = 101.25 mm (3.99 in), giving an overall diameter of 17 + (2 × 3.99) = 24.97 in. A 245/40R18 tire has a sidewall of 245 × 0.40 = 98 mm (3.86 in) and a diameter of 18 + (2 × 3.86) = 25.72 in. That's a difference of 0.75 in, or about 3% — right at the edge of the safe tolerance. At an indicated 60 mph, your true speed would be about 60 × (25.72 ÷ 24.97) ≈ 61.8 mph.

The tire size code is one of the most misunderstood specifications on a vehicle, yet it directly affects safety, performance, comfort, and efficiency. Reading a size like 225/45R17 94V is straightforward once you know the parts: 225 is the section width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of width), R indicates radial construction, 17 is the wheel diameter in inches, 94 is the load index, and V is the speed rating.

The aspect ratio has a bigger impact than most drivers realize. A lower profile tire (smaller aspect number) has a shorter, stiffer sidewall that improves cornering precision and steering response but transmits more road harshness into the cabin. A higher profile tire cushions bumps and better protects the wheel from potholes but can feel vague at the limit. This is why sporty cars use low-profile tires and comfort-oriented vehicles use taller sidewalls.

Plus sizing is the art of changing wheel and tire dimensions while keeping the overall rolling diameter constant. When you increase wheel diameter by an inch, you decrease the aspect ratio to compensate, keeping the tire the same overall height. Done correctly, plus sizing improves handling and appearance without affecting your speedometer, gearing, or ground clearance. This calculator makes it easy to find a plus-size combination that stays within tolerance.

Getting tire size wrong has real consequences. An oversized tire makes your speedometer read slower than your true speed — you might be going 65 when the dash says 60, risking speeding tickets and accelerated odometer wear. It can also cause the tire to rub during turns, reduce acceleration, and confuse safety systems that rely on wheel-speed data. Undersized tires cause the opposite problems. That's why the 3% diameter rule exists, and why checking a comparison before you buy is always worth the two minutes it takes.