Tire Size Comparison Calculator
Compare two tire sizes to check speedometer error, diameter difference, and fitment compatibility.
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Common Plus Sizing Examples
| Original | Plus 1 | Plus 2 | Diameter Match |
|---|---|---|---|
| 205/55R16 (24.9") | 215/45R17 (24.6") | 225/40R18 (24.7") | Close match |
| 225/65R17 (28.5") | 235/55R18 (28.2") | 245/45R19 (27.7") | Slight diff |
| 265/70R17 (31.6") | 275/60R18 (31.1") | 285/50R19 (30.6") | Truck sizing |
Tire Size Markings Decoded
A tire marked 225/65R17 means: 225 = section width in millimeters, 65 = aspect ratio (sidewall height as a percentage of width, so 225 × 0.65 = 146.25mm sidewall), R = radial construction, 17 = rim diameter in inches. The overall diameter is calculated as: rim diameter + 2 × sidewall height (converted to inches).
Why Speedometer Error Matters
Your speedometer is calibrated to the factory tire size. A taller tire travels more distance per revolution — the car physically moves faster than the speedometer reads. A shorter tire does the opposite. More than 3% error can trigger legal speeding violations, affect cruise control accuracy, and throw off odometer-based service reminders.
Plus Sizing Explained
Plus sizing means going up one rim size (e.g. 17" to 18") and reducing the aspect ratio to maintain approximately the same overall diameter. This improves steering response and handling because the shorter sidewall flexes less, at the cost of a slightly harsher ride. Always calculate diameter match when plus sizing to keep speedometer error under 3%.