A tire size comparison calculator shows the exact dimensional differences between two tire sizes. When upgrading or substituting tires, even small changes in overall diameter affect speedometer accuracy, odometer readings, and ground clearance. This tool quantifies all those changes instantly.
Enter Tire Sizes
Visual Profile Comparison
Dimension Comparison
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Speedometer & Odometer Impact
How to Compare Tire Sizes
Tire size comparison is essential before upgrading wheels or substituting tires. A 1% change in overall diameter may seem small but translates to measurable speedometer error and odometer inaccuracy. This tool decodes both tire sizes and shows every dimension that changes.
Understanding Tire Size Format
A tire size like 225/45R17 breaks down as: 225 = section width in millimeters, 45 = aspect ratio (sidewall height as % of width), R = radial construction, 17 = rim diameter in inches. The sidewall height is 225 × 0.45 = 101.25mm. Overall diameter = rim diameter in mm (17 × 25.4 = 431.8mm) + two sidewalls = 634.3mm or about 25 inches.
Speedometer Error Explained
Your speedometer measures wheel rotations per unit time and multiplies by the tire's circumference. If you install tires with a larger circumference than stock, each rotation covers more ground — so the speedometer undercounts your actual speed. At 60 mph, a 3% larger tire makes the speedometer show 58.2 mph while you're actually traveling at 60 mph. This is a safety issue at highway speeds.
Keeping Diameter Within Safe Limits
Most experts recommend keeping overall tire diameter within 3% of the OEM size. Beyond that, speedometer error becomes significant, ABS and traction control systems may be affected, and the tires may physically contact wheel wells during turns or suspension travel. The goal when plus-sizing is to use a wider tire with lower profile to maintain nearly identical overall diameter.
FAQ
Is this tire size comparison tool free?
Yes, completely free with no signup required. Enter two tire sizes in standard format (e.g., 225/45R17) to instantly compare dimensions and calculate the speedometer impact.
How do I read a tire size?
A tire size like 225/45R17 means: 225mm section width, 45% aspect ratio (sidewall height is 45% of width), R = radial construction, 17 inch rim diameter. The sidewall height in this example is 225 × 0.45 = 101.25mm.
What happens when you change tire size?
Changing tire diameter affects your speedometer accuracy, odometer reading, and ground clearance. Going to a larger diameter tire makes your speedometer read lower than actual speed (you're going faster than the gauge shows). A smaller tire makes it read higher (you're going slower).
How much speedometer error is acceptable?
Most countries allow up to 4% positive speedometer error (reading faster than actual speed). Negative error — where your speedometer shows slower than your true speed — is more problematic legally. Aim for less than 2% difference when changing tire sizes.
What is plus sizing in tires?
Plus sizing means fitting a larger rim diameter (e.g., going from 17 to 18 inches) while using a lower-profile tire to maintain the same overall diameter. This improves handling and appearance but can decrease ride comfort. Keep total diameter within 3% for minimal speedometer impact.
Is my data safe when using this tool?
Yes, all calculations run entirely in your browser. No information is ever sent to any server.
Can I put wider tires on my car?
Wider tires can improve grip but require sufficient fender clearance and may need different wheel offset. The tire width should generally not exceed the wheel width by more than 10mm. Always check manufacturer specifications for your specific vehicle.