Tire Size Calculator

About the Tire Size Calculator

A tire size calculator decodes the alphanumeric code on any tire sidewall (e.g., 225/65R17 92H), calculates the tire's overall diameter, sidewall height, section width, and circumference, and compares two tire sizes to show the percentage difference in speedometer reading accuracy, revolutions per mile, ground clearance, and gearing effect. Understanding tire size is essential when replacing tires to ensure proper fitment, when upgrading to larger wheels while maintaining the same overall diameter (plus-sizing), when installing a suspension lift with tires that won't rub, or when buying a used vehicle with non-factory tires that may have altered speedometer calibration. Our calculator also converts between metric tire sizing (most modern cars), flotation sizing (light trucks: 35x12.50R17), and LT metric sizing.

How It Works

Decoding 225/65R17: 225 = section width in millimetres (widest point of the tire). 65 = aspect ratio — sidewall height as a percentage of section width. Sidewall height = 225 × 0.65 = 146.25 mm. R = radial construction (virtually all modern tires). 17 = rim diameter in inches. Overall tire diameter = (2 × sidewall height in mm) + (rim diameter in mm). Rim in mm = 17 × 25.4 = 431.8 mm. Diameter = (2 × 146.25) + 431.8 = 292.5 + 431.8 = 724.3 mm = 28.52 inches. Circumference = π × diameter = π × 724.3 = 2,275 mm = 7.47 feet. Revolutions per mile = 5,280 feet / 7.47 ft = 707 rev/mile.

Tips & Best Practices

• ✓Speedometer accuracy: a larger overall diameter means your speedometer reads lower than actual speed. The calculator shows the exact percentage error and correction factor for any size change.
• ✓Plus-sizing rule: increasing wheel diameter by 1 inch while decreasing aspect ratio maintains similar overall diameter and preserves speedometer accuracy. e.g., 225/65R17 → 235/55R18 (nearly equal diameter).
• ✓Rub and clearance: tire diameter increase raises ride height by half the diameter increase. A 2-inch diameter increase = 1 inch more ground clearance (and 1 inch less clearance to wheel wells and suspension).
• ✓Load index: the load index number in the tire code (e.g., 92 in 225/65R17 92H) indicates maximum load capacity (92 = 630 kg per tire). Never install tires with a lower load index than OEM specification.
• ✓Speed rating: the letter after load index (H = 130 mph, V = 149 mph, W = 168 mph, Y = 186 mph). Choose a speed rating at or above OEM specification for safety and warranty compliance.
• ✓Seasonal tires and size: winter/snow tires are often narrower than summer tires (lower section width, higher aspect ratio) to improve snow penetration and reduce hydroplaning. A 225/65R17 summer tire might pair with a 205/70R16 winter tire on a smaller winter wheel.
• ✓Flotation sizing (off-road trucks): 35x12.50R17 means 35-inch overall diameter, 12.5-inch section width, R17 rim. The calculator converts between flotation and metric sizing.
• ✓Gearing effect: larger tires reduce effective gear ratio (like adding a higher gear), decreasing acceleration and fuel economy for the same engine output while increasing top speed. A 3% diameter increase ≈ equivalent to lowering the gear ratio by 3%.

Who Uses This Calculator

Vehicle owners comparing tire options and ensuring fitment before purchase. Mechanics and tire shops advising customers on size upgrades and equivalents. Off-road enthusiasts calculating how large a tire will fit their lift kit. Used car buyers verifying that installed tires match factory specs or identifying if the speedometer is uncalibrated. Fleet managers standardising tire specifications across vehicles. Auto enthusiasts planning plus-size wheel upgrades while maintaining OEM ride characteristics.