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Tire Size - 205/60 – 14:

205 The approximate cross section width of the tire in millimeters.

60 The ratio between the height of the tire and the cross section width. The sidewall of this tire is 60% as high as it is wide. A smaller aspect ratio generally provides a stiffer sidewall with more sporting handling characteristics and some deterioration in ride quality.

14 The outside diameter of the wheel.

Calculating Overall Tire Diameter
With this formula you can figure out how to fit a wider, lower profile tire with little change in overall diameter. Thus you can improve grip while retaining speedometer accuracy.

The formula for overall tire diameter is:

As an example, let’s use the original tire size on a Giulietta: 155/78 – 15.

155 Approximate cross section width in millimeters.
78 Aspect ratio which is converted to “0.78”
15 Wheel diameter in inches.


The tire size we would like to use is 185/65 – 15. If we plug in the new tire size into our formula, we get:

These two diameters are very close, so speedometer accuracy and final drive ratio would be unchanged by changing from a 155/78 – 15 to a 185/65 – 15 but the tire contact patch would be much enlarged for improved handling and braking.

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