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.
Comparing a current 225/45R17 tire against a replacement 235/40R18: current overall diameter = 17 × 25.4 + 2 × (225 × 0.45) = 24.97 inches, new overall diameter = 18 × 25.4 + 2 × (235 × 0.40) = 25.40 inches, a difference of about +1.72%. Since the new tire is slightly larger, revolutions per mile drop slightly (807.6 to 794.0), and your speedometer would under-read by about 1.72%, showing roughly 68.8 mph when you're actually going 70 mph, comfortably inside the commonly cited 3% safe-change guideline.
Misreading the direction of the speedometer error, a larger new tire (positive percentage) makes your speedometer under-read, showing a lower speed than you're actually going, it's easy to assume the opposite intuitively.
Only checking diameter difference and skipping load rating, speed rating, and physical clearance, this calculator compares dimensions only, a tire that passes the 3% diameter guideline can still be the wrong load or speed rating, or physically rub against the fender or suspension.
Assuming aftermarket wheels with a different rim diameter always keep the same overall tire diameter, section width and aspect ratio both change the sidewall height too, plugging in a new rim size without recalculating the full tire size notation can produce a much bigger diameter change than expected.
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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