Compare your current tire size against a replacement size to see the change in overall diameter, revolutions per mile, and speedometer accuracy.
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A tire size like 225/45R17 breaks down as: 225 = section width in millimeters, 45 = aspect ratio (sidewall height as a percentage of section width), and 17 = rim diameter in inches. This calculator uses those three numbers for both your current and replacement tire to compute each tire's overall diameter: Rim Diameter (converted to mm) + 2 × Sidewall Height, then converts back to inches.
Your speedometer and odometer are calibrated based on how far your original tires travel per wheel revolution. A replacement tire with a larger overall diameter travels farther per revolution, causing your speedometer to under-read your actual speed (and your odometer to under-count actual distance), while a smaller replacement tire causes the opposite, an over-read.
A common rule of thumb is keeping the overall diameter change within about 3%, to avoid meaningfully affecting speedometer accuracy, clearance, and the accuracy of safety systems like ABS that rely on wheel speed sensors. Always check your vehicle manufacturer's guidance before changing tire sizes.
No, this calculator compares physical dimensions only. Load rating, speed rating and vehicle clearance are separate considerations that should also be checked before fitting a different tire size.
A negative percentage means your new tire is smaller in diameter than your current tire, causing your speedometer to over-read, showing a higher speed than you're actually traveling.
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