Tire Diameter Calculator

01

Total tire diameter is the distance from one outer edge to the other. The formula is Total Diameter = Wheel Diameter + (Sidewall Height × 2). For example, a 225/45R17 tire: Width 225mm, Aspect ratio 45%, Wheel 17 inches. Sidewall height = 225mm × 0.45 = 101.25mm. Wheel diameter = 17 inches × 25.4mm/inch = 431.8mm. Total diameter = 431.8mm + (101.25mm × 2) = 634.3mm ≈ 63.4cm. Tire circumference = π × total diameter = 3.14159 × 634.3mm = 1992mm ≈ 2m. This circumference is the distance the vehicle travels per tire revolution, directly affecting speedometer and odometer accuracy.

02

Changing total tire diameter causes speedometer error. Vehicle speedometers calculate speed based on tire rotations. Larger tires result in actual speed exceeding indicated speed; smaller tires show the opposite. Example: Original diameter 634mm (225/45R17) → Replacement 660mm (235/50R18) = +26mm increase. Diameter increase = 26 ÷ 634 = 4.1% increase. When speedometer shows 100km/h, actual speed is 104.1km/h. Conversely, 634mm → 608mm (205/45R17) = -26mm decrease. At indicated 100km/h, actual speed is 95.9km/h. Acceptable range: Generally maintain within ±3% for safety and legality. ±5% or more may fail vehicle inspection. Odometer error: 4% larger diameter means actual 10,000km shows only 9,600km on odometer.

03

When replacing tires, maintaining original total diameter is crucial. Plus-sizing example: 16 to 17 inch upgrade. Original 205/55R16 total diameter = 631.7mm. Goal: Upgrade to 17 inch while maintaining 631.7mm diameter. 17-inch wheel diameter = 431.8mm, required sidewall height = (631.7 - 431.8) ÷ 2 = 100mm. Required aspect ratio = 100 ÷ tire width × 100. Selecting 215mm width: aspect ratio = 100 ÷ 215 × 100 = 46.5% ≈ 215/45R17 (total diameter 631.5mm, error 0.2mm). Minus-sizing example: 17 to 16 inch. Original 225/45R17 diameter = 634.3mm. 16-inch wheel diameter = 406.4mm, required sidewall = (634.3 - 406.4) ÷ 2 = 114mm. Selecting 205mm width: aspect ratio = 114 ÷ 205 × 100 = 55.6% ≈ 205/55R16 (diameter 631.7mm, error -2.6mm). Practical tip: Use online tire diameter calculators to test various combinations, selecting sizes within ±5mm diameter error.

04

Plus-sizing advantages: Sporty, premium appearance. Improved cornering and agile steering response. Enhanced traction reducing braking distance. Plus-sizing disadvantages: Harsher ride feeling more road impacts. Higher tire and wheel costs (17→18 inch: 20-30% increase per tire). Increased wheel damage risk (speed bumps, potholes). Slightly reduced fuel economy (added weight, rolling resistance). Minus-sizing advantages: Smoother ride with better shock absorption. Lower tire and wheel costs. Better for winter tires (improved snow traction). Slightly improved fuel economy. Minus-sizing disadvantages: Less sporty appearance. Reduced cornering performance. Potentially decreased high-speed stability. Recommendation: Minus-size for winter (frequent urban driving), plus-size for summer (highway and winding roads).

05

Changing total tire diameter alters circumference, affecting engine RPM and gear ratios. Assume original circumference 2000mm, 100km/h requires 2500 RPM. Increasing diameter 5% to circumference 2100mm: Same 100km/h requires tire rotations = original rotations ÷ 1.05 = 95.2%. Engine RPM = 2500 × 0.952 = 2380 RPM. Lower RPM at same speed slightly improves fuel economy but reduces acceleration. Conversely, reducing diameter 5% raises engine RPM, improving acceleration but increasing fuel consumption and noise. Transmission stress: Significant diameter changes may disrupt automatic transmission shift timing, causing shift shocks. 4WD/AWD caution: Front-rear tire diameter difference exceeding 5mm strains center differential, risking failure. Replace all four tires with same size.

06

When buying used cars: Verify tire sizes are OEM. Compare with recommended sizes on driver door jamb sticker. If non-OEM tires are installed, calculate total diameter to confirm within ±3%. Large differences may cause speedometer errors, transmission issues, insurance problems. Selecting winter tires: Recommend same size or minus-sizing for winter. Example: Summer 225/45R17 → Winter 205/55R16 (similar total diameter). Better snow traction at lower cost. Spare tires: Temporary spares (donut tires) have smaller diameter, limiting speed to 50km/h for 80km maximum. Full-size spares must match OEM size. Tire pressure: Slightly increase pressure (+0.2bar) for larger diameter, decrease (-0.2bar) for smaller. Tire wear: Tread wear reduces total diameter. At 50% wear, diameter decreases ~5mm, speedometer reads ~1% faster.