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What is the calculation formula for tire outer diameter size?

3 Answers

LaLuna

09/06/25 3:33am

The calculation formula for tire outer diameter D is: D = tire diameter + tire width * aspect ratio * 2. The following introduces the functions of car tires: 1. Supporting weight: Supporting the entire weight of the vehicle, bearing the load of the car, and transmitting forces and moments in other directions. 2. Cushioning impact: Transmitting traction and braking torque, ensuring good adhesion between the wheels and the road surface to improve the car's power, braking, and passability; working together with the car suspension to cushion the impact during driving and attenuate the resulting vibrations. 3. Ensuring car safety: Preventing severe vibrations and early damage to car components, adapting to the vehicle's high-speed performance and reducing driving noise, ensuring driving safety, handling stability, comfort, and energy-saving economy.

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Reid

09/18/25 3:32am

I'm very meticulous when it comes to changing tires and always calculate the outer diameter myself. The formula is quite simple: (Tire width in mm × Aspect ratio percentage × 2 ÷ 1000) + (Wheel diameter in inches × 25.4), with the final unit being millimeters. For example, a 215/60 R16 tire would be calculated as 215×0.6×2÷1000 +16×25.4, resulting in approximately 664mm. The key point is that the diameter difference between new and old tires shouldn't exceed 3%, otherwise the speedometer will be inaccurate and there might be wheel arch rubbing. Last week when helping a neighbor calculate modified tire sizes, I found that 20-inch wheels with 35 aspect ratio tires were 5% smaller than the factory specifications, so I quickly advised against it to avoid annual inspection issues.

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VivianLynn

11/01/25 3:57am

I specifically researched the calculation logic when changing my son's tires. Outer diameter = rim diameter × 25.4 + 2 × tire width × aspect ratio / 100. Note that the width unit is millimeters, and the rim uses inches. The key to the formula is understanding the tire sidewall height: the 60% in 215/60 is actually 60% of the 215mm width. During the last vehicle inspection, I found that the modified tires were 12mm smaller than the original ones, which caused the ABS warning light to come on due to the excessive wheel speed difference. It's recommended to compare several specifications during calculation, and it's best to be cautious if the sidewall height change exceeds 3cm.

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