Tire Size Converter

A tire size designation like 225/45R17 encodes three critical dimensions: the tire's section width, aspect ratio, and rim diameter. The first number (225) is the section width in millimeters — the distance from sidewall to sidewall when the tire is mounted and inflated. The second number (45) is the aspect ratio, expressed as a percentage of the width — this means the sidewall height is 45% of 225 mm, or 101.25 mm. The final number (17) is the rim diameter in inches.

Understanding these dimensions is essential when changing tire sizes, whether for performance upgrades, winter tire fitment, or replacing damaged tires. Changing any of these three parameters affects the tire's overall diameter, which in turn affects speedometer accuracy, odometer readings, gear ratios, and handling characteristics.

The overall diameter of the tire is calculated as: rim diameter + 2 × sidewall height. For a 225/45R17 tire: sidewall height = 225 × 0.45 = 101.25 mm, overall diameter = 17 inches + 2 × 101.25 mm. Converting the rim to millimeters (17 × 25.4 = 431.8 mm) and adding 2 × 101.25 = 202.5 mm gives a total diameter of 634.3 mm, or about 24.97 inches.

This calculator not only computes tire dimensions but also compares your current tire size against common alternatives, showing the speedometer error that would result from each substitution. Keeping the overall diameter within 3% of the original is the general guideline for safe tire substitution.

The fundamental tire dimension formulas derive directly from the tire size designation:

Sidewall Height = Section Width × (Aspect Ratio / 100). This gives the distance from the rim to the outer tread surface in millimeters. To use this in diameter calculations with the rim measured in inches, convert to inches by dividing by 25.4.

Overall Diameter = Rim Diameter (inches) + 2 × Sidewall Height (inches). The overall diameter determines the tire's circumference and thus the number of revolutions per mile or kilometer.

Circumference = Overall Diameter × π. This is used to calculate revolutions per distance: Revolutions per mile = 63,360 / Circumference (in inches), and Revolutions per km = 39,370 / Circumference (in inches).

When you change tire size, the speedometer calibration becomes inaccurate because the speedometer is calibrated to the original tire's revolutions per mile. A tire with a larger overall diameter rotates fewer times per mile, meaning the vehicle travels farther per rotation than the speedometer expects. This causes the speedometer to read lower than the actual speed.

Conversely, a smaller tire diameter results in more rotations per mile, causing the speedometer to read higher than actual speed. The percentage error is calculated as: Speed Error (%) = ((New Diameter / Original Diameter) - 1) × 100. For example, if you increase tire diameter by 3%, your speedometer will underread by approximately 3% — showing 60 MPH when you are actually traveling at 61.8 MPH.

The general guideline is to stay within 3% of the original overall diameter to maintain acceptable speedometer accuracy and avoid significant changes to gearing, braking, and handling characteristics. This calculator shows the speed difference for several common tire size alternatives, helping you make informed decisions when considering tire size changes.

This calculator provides comprehensive tire size analysis:

1. Enter Your Tire Size: Input the section width (mm), aspect ratio (%), and rim diameter (inches) for your current tire.
2. View Calculated Dimensions: The calculator shows overall diameter, sidewall height, circumference, section width in inches, and revolutions per mile/km.
3. Review Size Comparison: The comparison table shows several common tire sizes, their diameters, and the speedometer error relative to your current tire. Green values indicate larger tires; red values indicate smaller tires.
4. Quick Select Common Sizes: Click any of the preset buttons (Compact Car, Sedan, Sports Car, SUV, Truck, Performance) to instantly load a common tire size for comparison.

Tire size conversion is essential in automotive maintenance and repair. When original tire sizes are unavailable or when upgrading to performance or winter tires, understanding dimensional equivalence ensures safe fitment. A tire shop must verify that replacement tires have compatible overall diameter, width, and load rating.

In motorsport and performance tuning, tire size selection directly affects acceleration, top speed, and handling. Lower-profile tires (lower aspect ratio) provide sharper steering response but less sidewall cushioning. Wider tires offer more grip but increase rolling resistance and fuel consumption.

Off-road vehicle customization often involves fitting larger tires for increased ground clearance and traction. This calculator helps off-road enthusiasts understand the speedometer and gearing implications of lifting a vehicle and fitting larger tires, enabling informed decisions about gear ratio changes.

When buying a used vehicle, the tires may not match the factory specification. This calculator helps verify whether the installed tires are within acceptable parameters or whether speedometer corrections or gear ratio adjustments may be needed.