Tire Size Calculator
Compare two tire sizes: overall diameter, the difference in millimeters and percent, and the real speed when your speedometer reads 60 mph.
Formula
Overall diameter (mm) = wheel diameter (in) × 25.4 + 2 × section width × aspect ratio ÷ 100.
Example: a 205/55 R16 tire is 16 × 25.4 + 2 × 205 × 0.55 = 406.4 + 225.5 = 631.9 mm in diameter.
Frequently asked questions
How do I read a tire size such as 205/55 R16?
205 is the section width in millimeters, 55 is the sidewall height as a percentage of the width (the aspect ratio), R means radial construction, and 16 is the wheel diameter in inches.
How much difference in diameter is acceptable?
Follow the sizes in your owner's manual or on the placard in the driver's door jamb. As a rough rule of thumb, keeping the overall diameter within about 2 to 3 percent of the original avoids large speedometer and gearing errors.
Why does my speedometer read differently after changing tires?
The speedometer counts wheel rotations. A taller tire covers more road per rotation, so the car moves faster than the display shows. A shorter tire does the opposite.
Does it check whether the tire will fit?
No. It compares diameters only. It does not check wheel width, wheel-well clearance, load rating or speed rating.
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For informational purposes only. Check tire and wheel fitment with your vehicle's documentation or a tire professional.