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Tire Size Calculator

Compare two tire sizes instantly. Check diameter differences and speedometer error.

1 Current Tire

2 New Tire

Comparison Results

Target calculated
Metric Tire 1 Tire 2 Difference
Diameter - - -
Width - - -
Sidewall - - -
Circumference - - -
Revs/Mile - - -
Speedometer Error Safe
Reading
60 mph
Actual
60.0 mph

Speedometer reading is accurate.

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Tire Sizes Explained: Reading the Sidewall

Ever wondered what all those numbers on your tire mean? Understanding tire sizes is crucial when buying replacements or upgrading your wheels.

Let's break down a typical size like 205/55 R16:

205 (Width)

The first number is the width of the tire in millimeters, measured from sidewall to sidewall. A wider tire generally offers better grip but may increase fuel consumption.

55 (Aspect Ratio)

This is the height of the sidewall as a percentage of the width. In this case, the sidewall height is 55% of 205mm. Lower numbers mean a shorter / "low profile" sidewall.

16 (Diameter)

The last number is the diameter of the wheel rim in inches. You can only mount a 16-inch tire on a 16-inch rim. This is critical for fitment.

Why Speedometer Error Matters

When you change tire sizes, you alter the overall diameter of the wheel. Since your car's speedometer calculates speed based on wheel rotations, changing the diameter affects the reading.

  • Larger Diameter: Your speedometer will read slower than your actual speed. You might be speeding without knowing it.
  • Smaller Diameter: Your speedometer will read faster than your actual speed. You'll pile up odometer miles faster than actual.

Frequently Asked Questions

Most experts recommend staying within 3% of the original tire diameter. Anything more can significantly affect your speedometer, odometer, and even safety systems like ABS and Traction Control.
It depends on your rim width. Most rims can accommodate a small range of tire widths (e.g., a 7-inch rim can usually take 195mm to 225mm tires). Check your rim manufacturer's specs.
Yes. Wider tires have more rolling resistance, which can slightly decrease fuel economy. Larger diameter tires effectively change your gear ratio, which can also impact city vs. highway mileage.

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