Tire Fitment Calculator

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Fact-checked by David Chen, Automotive Engineering Consultant • Updated Oct 2023

Compare tire sizes to see how a new fitment affects your vehicle’s diameter, speedometer accuracy, and clearance. Essential for wheel upgrades and aftermarket modifications.

Tire Fitment Calculator

Current Tire Size

New Tire Size

Fitment Comparison Result

Tire Fitment Calculator Formula:

Total Diameter = (2 × Width × Aspect Ratio / 100) + (Rim Size × 25.4)

Source: Tire Rack Engineering Standards and Discount Tire Education.

Variables:

  • Width (mm): The width of the tire from sidewall to sidewall in millimeters.
  • Aspect Ratio (%): The height of the sidewall as a percentage of the width.
  • Rim Diameter (inches): The size of the wheel the tire is mounted on.
  • Speedometer Error: The percentage difference in circumference affecting speed readings.

Related Calculators:

What is a Tire Fitment Calculator?

A tire fitment calculator is a specialized automotive tool used to compare the physical dimensions of two different tire and wheel combinations. It helps car enthusiasts and mechanics determine if an aftermarket tire size will fit within the wheel well and how it impacts the vehicle’s driving characteristics.

Proper fitment is crucial for safety. When you increase the wheel size or change the tire width, you must ensure the overall diameter stays within a 3% variance of the original size to prevent transmission issues and significant speedometer inaccuracies.

How to Calculate Tire Fitment (Example):

Let’s say you are upgrading from 205/55R16 to 225/45R17:

  1. Calculate Sidewall Height: 205mm × 0.55 = 112.75mm.
  2. Convert Rim to mm: 16 inches × 25.4 = 406.4mm.
  3. Calculate Total Diameter: (112.75 × 2) + 406.4 = 631.9mm.
  4. Repeat for New Tire: (225 × 0.45 × 2) + (17 × 25.4) = 634.3mm.
  5. Compare: The difference is 2.4mm (approx 0.38%), which is a safe fitment.

Frequently Asked Questions (FAQ):

Is a 3% difference in tire size safe? Most experts recommend staying within a 3% range of the original diameter to avoid stressing the drivetrain and brakes.

How does a bigger tire affect my speedometer? A larger diameter tire travels further per revolution. This means your speedometer will show a lower speed than you are actually traveling.

Will wider tires improve grip? Generally yes, as they provide a larger contact patch, but they can also increase hydroplaning risk and fuel consumption.

Do I need new wheels for a tire size change? Only if the rim diameter changes or the new tire width is outside the rim’s recommended width range.

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