Metric Tiger Paws

By The Metric Maven

Bulldog Edition

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.[6]


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 and contribute. Also purchase his books about the metric system:

The first book is titled: Our Crumbling Invisible Infrastructure. It is a succinct set of essays  that explain why the absence of the metric system in the US is detrimental to our personal heath and our economy. These essays are separately available for free on my website,  but the book has them all in one place in print. The book may be purchased from Amazon here.

The second book is titled The Dimensions of the Cosmos. It takes the metric prefixes from yotta to Yocto and uses each metric prefix to describe a metric world. The book has a considerable number of color images to compliment the prose. It has been receiving good reviews. I think would be a great reference for US science teachers. It has a considerable number of scientific factoids and anecdotes that I believe would be of considerable educational use. It is available from Amazon here.

The third book is called Death By A Thousand Cuts, A Secret History of the Metric System in The United States. This monograph explains how we have been unable to legally deal with weights and measures in the United States from George Washington, to our current day. This book is also available on Amazon here.

10 thoughts on “Metric Tiger Paws

  1. Oddly, even the European manufacturers use inch-designated rims. The metric-designated rim you refer to had a completely different bead seat design such that tires for the two systems are completely incompatible. The Metric designated diameter tires were so expensive that most owners bought new wheels and mounted conventional tires when their original tires wore out. Footnote 6 of your Wikipedia articles alludes to this.

    The wheel size is a basic parameter in a spec that details many parameters of the wheel, particularly the bead seat design, Inch-based sizes refer to one bead seat design, the one commonly used, and the metric to one nearly abandoned.

    Of course a new wheel rim spec could be written all in metric, but no one seems enthused about doing so, It is somewhat like the foot in aviation (for altitude).

  2. I have a tire gauge for automotive use that, similarly to auto speedometers, is round with the outdated units on the outside and metric units, in kPa, on the inside of the dial. When I check the pressure, which I do at least weekly, I use the range of 220 kPa to 240 kPa to be within the limits of the correct pressure. Above or below these limits I adjust as necessary using my small air compressor. My bicycle pump has a similar pressure gauge that is built in. The metric units of this dial is given in bars. I use 40 as the correct bar and adjust as necessary. When I need a tool, I always try to find one with pure metric units only. This is rarely the case, unfortunately.

    There is movement to metric in the medical field. My wife had sinus surgery yesterday and her weight and height were solely in metric units on the forms, including the ones we were given to bring home.

    • Funny thing about American tires is that they are marked with their max pressure in kilopascals first and pounds in parentheses. The load in kilograms also comes first.

  3. I remember the British having a similar “penny test” introduced in the late 1960s. The minimum tread requirement was one millimetre, if I recall correctly.

  4. Peter G.

    I doubt 1 mm is correct. On the US penny, the words “In God We Trust” are written above Lincoln’s head. There are numerous jokes that if you can read the words, you are counting on divine intervention to support your tires.

    I think all tires now have tread wear indicators anyway. When the wear of the black tread uncovers the colored tread wear indicator, it is time for new tires.

    • I don’t know when wear indicators were first built into tyres. I do remember the “penny test” being advertised on British TV. They showed a penny with a series of dots inside its outer rim, as per these pictures:

      If you inserted the coin into the tyre tread and couldn’t see the dots it was time to replace the tyre.

  5. You said:
    “It was down to about 20 PSI (138 Kilopascals) the normal value is about 32 PSI (221 Kilopascals).”

    Why did you convert ABOUT 20 psi to 138 kPa and ABOUT 32 psi to 221 kPa instead of 140 kPa and 220 kPa? Have you ever looked closely at the metric tires on your car and noticed that the kilopascals (the “k” is not capitalized) come first and are round with the last digit a zero and the “psi” is in the after thought position?

    There is no need to convert USC to exact metric values. You can round aggressively so the metric value comes out clean and rounded. Especially when the original measurement is not exact but defined as “about”.

    Also, if you express a unit as psi using symbols, then why spell out kilopascals instead of using kPa? Others who see this will get a bad impression about SI. To them USC is easier because one has to think of three letter, p-s-i, but with metric we have this long 10 letter word.

  6. I’m surprised you have just discovered the P-metric tire series. They have been around since the ’70s. I never owned a car that did not have them and I owned my first car in 1978.

    The inch rims are made in metric and only the one dimension is a conversion of an inch value, though it may not be exactly that as there is a +/- 2 mm tolerance on that particular rim dimension. A 15 inch rim dimension could easily be 380 mm.

    “A load index table tells you that 92 actually means 1389 pounds.”

    If you had stated it properly in kilograms, you would have seen a not so round pounds is exactly 630 kg. If you would have looked even closer at the load index chart, you would have noticed that all of the values are rounded metric first with numbers ending in a zero or a five and the pounds are an afterthought with numbers all over the place.

    Yes, the tire and rim is 99.999999999 % metric with the only reference to inches being an obscure number that on a real technical drawing appears in millimetres +/- 2 mm tolerance.

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