Nuts I M Confused

LT285/75R16. We know the R16 means it is a radial tire on a 16-inch rim, just like our inch-sized tire. (It's weird that they measure the rim in inches but the rest of the tire In millimeters!) But what is all this LT285/75 stuff?

LT means the tire is for a light truck, while P in that position would mean a passenger vehicle. The larger number, 285, is the section width in millimeters. So to determine the width in inches, simply divide by 25.4, the conversion of any millimeter measurement into inches. This still leaves us with overall height to figure out, and usually that's the number we want.

The 75 is called the aspect ratio. It is the height of the tire expressed as a percentage of the width. We know the width is 285 mm, so if the height is 75 percent of that, the height is 213.75 mm (285 x 0.75). But that is just for the height from the tread to the rim, so we need to double that, and we get 427.5 mm. Then we divide that by 25.4 and it changes to inches: 16.83. Then we add the diameter of the wheel (16) and get 32.83, or roughly 33 inches.

Thoroughly confused? Just remember "SA2 by 25.4 plus rim." That means section width (S) times aspect ratio (A) times 2 divided by 25.4 plus rim diameter. The result is tire diameter.

Thanks for the great question. Your letter was picked as this month's Nuts,

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