Tire Diameter for Dummy Drivers

Tire Rolling Diameter

Although most of our DotD nominees are featured for foolish mistakes (most related to ignorance and/or inability to copy-reword-paste Wikipedia), more than a few of them are self-described "professional writers" who dip their toes in the wrong subject. Today's nominee is a case in point; Mark Kennan is a business writer who knows so little about the automotive industry that he tried to turn "How to Calculate Tire Turns Per Mile" into a math question for Sciencing.com.

In reality, it's not. Because tires are made of rubber and rubber is flexible, the weight of the vehicle always "squashes" the tire out of round. It's flat on the bottom, in other words. So manufacturers must either put the tire a dynamometer to count revs/mile, or use some pretty sophisticated math.

Kennan (sometimes known as Keenan) didn't approach the question from that direction, instead he treated the tire as if it were a rigid disc. According to Mark, all you need do is [M]easure the diameter of the tire in inches... [M]ultiply the diameter by pi... [D]ivide 63,360... by the tire circumference... Which, we admit, would tell you the number of revolutions per mile of a wooden wagon wheel or the steel wheel of a locomotive. It will not, however, tell you the number of revolutions per mile of a pneumatic tire. For the record, it will underestimate the count.

We noticed, too, that Kennan was apparently ignorant of the process of determining the rolling diameter of a tire. He wants you to "measure" the diameter, which seems rather clumsy (though not as clumsy as measuring the circumference). No, all you need to do to determine the rolling diameter of a tire is to read the sidewall. Here's how, using a tire size of P215/65 R 15:

1. The wheel diameter is 15.
2. The tire width is 215mm (8.46 inches)
3. The tire's height is 65% of 215mm, or 139.75mm (5.5 inches)
4. The tire's rolling diameter is the wheel diameter plus two times the tire height: 15 + 11 = 26 inches.

Once that's over, you can get a "perfect-world" estimate of revolutions per mile: it's a minimum of 63,360 / 26π or about 776 turns per mile. Since you have no idea of the tire inflation, the vehicle's weight, or the tensile strength of the sidewalls, you can't actually calculate the number, but this is a start. According to Kennan – who apparently doesn't know how to read tire size and didn't bother to research the question at all – it's "the answer." No wonder he's our Dumbass of the Day.

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