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Friday, December 12, 2008

Right Tyre Formula

Appreciate if you could provide the formula for the 3% rule on changing tyre size. I remembered that you had published this formula sometime back but unortunately I did noyt keep it.

I wanted to change my tyre from 175/65 R14 to 185/60 R14 or 195/60 R14 but i can't remember the formula to check whether this is within the 3% rule.

Reply:
To calculate the overall diameter of the tyre:
The formula is: (D x 25.4) + 2(S x A)
D= Rim diameter in inches
S= Tyre width in mm
A= Aspect ratio in decimals

If the tyre is 165/65 R14:
The overall diameter would be (14 x 25.4) + 2(165 x 0.65) = 570mm

For the 195/60 R14
The overall diameter would then be (14 x 25.4) + 2(195 x 0.60) = 589mm

The difference is 19mm which is 3.33%, a bit over the recommended 3%.

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