By The Metric Maven
I recently took my car in for an oil change, and to have the dealer look it over and verify it as roadworthy for a trip back to the midwest. Above the desk of the person helping me was a flat panel monitor which was explaining “The Penny Test” for tires. Public service commercials years ago showed that if you put a penny into a tire tread with the top of Abraham Lincoln’s head in the tread, and the top of his head was not covered, the tires were worn and needed replacing. This video showed different lines along the coin, gave a value in terms of 1/32 of an inch and had red, yellow and green backgrounds for each value. I shook my head and thought: “Wow, that’s crazy, who knows how much 3/32ths of an inch is or how it compares to 7/32nds? Millimeters would actually mean something.”
I sat and waited for them to finish. A pleasant woman came with the inspection results. She showed me that my brakes were mostly ok, but close to needing to be changed on the front as there was only 3 mm left. On the back it was about 4 mm and could also wait. Ah, millimeters, no 32nds of an inch. But there was also some bad news. I had recently purchased new tires, and one of them had a screw puncturing the top, and a nail in the sidewall. The tire could not be fixed because of this. It was slowly leaking. It was down to about 20 PSI (138 Kilopascals) the normal value is about 32 PSI (221 Kilopascals).
I immediately drove my car to the local tire shop where I had purchased the tires. To my amazement, the tire was in warranty. The bad news?—the tire had been discontinued. This was the third time Michelin had discontinued the recommended tire. As I needed to go on a trip, I put a very similar tire on. The person helping me indicated that a new tire had been designed to replace the old version, but it was not of a compatible size. The next time I was in and needed all new tires, he suggested I purchase the new tire model. I asked what the difference was. I was told one important change was that it is 10 millimeters narrower than the other design. My mind screeched to a halt.
“Ten millimeters?—are tires all metric?”
“Well, mostly, most all of them are now.”
I asked the technician what he meant. He then escorted me over to a nearby wheel rim to explain. He pointed out that a 14 inch rim is measured from the diameter of the bead (the seal) and the width of the rim is also in inches. The bolt pattern is in metric, and the standoff of the mounting plate for the rim is in millimeters, in this case it was 41 mm. I asked if the bolts were all metric.
“Well, mostly, they are M12 x 0.5 but on older types of wheels like those still used on campers and trailers they are often 9/16″ and the wheels are all in standard.”
AHHHHHH!….there it was…that word standard again, for barleycorn inches. I told him “well, it’s standard for 5% of the worlds population.”
He smiled and said “only we could make it this complicated.”
How could I argue with that? The technician then explained that for common passenger tires one can read the set of numbers found on its side and determine important properties of the tire.
The designation I saw on a tire in the show room was 215/55 R16 97H. The 215 means the width of the tire is 215 millimeters. The number after the slash is the aspect ratio which
is 55. So the height of the sidewall is 0.55 X 215 mm or 118.25 mm. So far so good. The R means radial tire and the 16 means sixteen inches. So the tire designation is not all metric, but is a pigfish combination. Oh…the pain. Clearly with all the new cars, and new tires that have been designed over the years, the radius could have been changed to 400 mm with little problem. What I saw was that all the sales literature is in inches for the rims and tires. One would never see a millimeter where showroom information met the American consumer.
The number 97 is the load index of the tire, which in the typical indirect designation of which Americans make a fetish (like gauge numbers), it does not correspond directly to any known units. A load index table tells you that 92 actually means 1389 pounds. This sort of irrational designation is what makes America great! My mind kept nagging me with hope that perhaps the radius designation in inches is actually a metric value that was converted back to inches and rounded. I consulted Wikipedia about tire code, and unfortunately this appears not to be the case:
- 2 digit number: Diameter in inches of the wheel that the tires are designed to fit. There is the rare exception metric diameter tires, such as the use of the 390 size, which in this case would indicate a wheel of 390 mm in diameter. Few tires are made to this size presently.
With little else to do while waiting for the tire to be mounted, I asked about the “penny test” and the 32nds of an inch. I was told the “penny test” was mostly out of favor these days. I asked what the values were for red, yellow and green for tire tread in 32nds of an inch. There seemed to be some uncertainty. Finally one of the attendants tossed a gauge in front of me and said “here, you can keep this, it will tell you.” Indeed it does. It is a six sided plastic polygon cylinder which has a scale in 32nds on it. Here is what it states:
0-3 32nds is Red
3-6 32nds is Yellow
6-32 32nds is Green
It was clear to me they used the gauge constantly, but the numbers on it remained foreign. This is probably because 1/32″ is not exactly a common unit and they just look at the color on the gauge. This caused me to look back at the print out I was given by my car dealer for the brakes as it had the same colors. When I looked, I was surprised. Here is the top line:
I had completely missed the tire tread data of 7/32nds for my tire tread. There it is, side by side, fractions of inches and millimeters. There is not even a designation for inches, it just has fractions! It also appears to disagree with the gauge I was given at the tire store–by 1/32 of an inch.
Let’s see how millimeters might work. For the tire shop:
0 to 2.5 mm is Red
2.5 mm to 5 mm is Yellow
5 mm to 25 mm is Green
For my car dealer:
0 to 3.0 mm is Red
3.0 mm to 5 mm is Yellow
5 mm to 25 mm is Green
Either set of values seems simple, provides dimensional meaning, and is easy to remember when compared with fractions of an inch. So, does having tread thickness in thirty-seconds of an inch make the tread thickness more understandable because it’s in “standard” units? I don’t see how having two sets of units, where one uses fractions and the others decimals makes any sense. Perhaps that’s why we in the US do this, to obscure any rational understanding.
Cars may be over 99% metric, but until the US switches and industry is compelled to exclusively use the metric system for commerce, there will never be 100% anything in the US, other than confusion.
If you liked this essay and wish to support the work of The Metric Maven, please visit his Patreon Page.