Try This Tire on for Size

Dear Car Talk

Dear Car Talk | Dec 13, 2019

Dear Car Talk:

Can you tell me why tire sizes are designated as they are?

If I understand correctly, we have metric, English and a ratio. Like, a "235-75R15" tire is 235 millimeters wide, has a 15-inch hole in the middle for the wheel, and the sidewall height is 75 percent of the tire's width.

Can you explain the rationale behind all these different measurement systems? It seems like a Brit, an American and a statistician walked into a bar ... -- John

Great question, John. And not an easy one to answer definitively.

The general answer is that the U.S. has stubbornly held onto its beloved feet and inches while the rest of the world has been trying to nudge us into meters and millimeters. And because that nudging has been only partly successful, we've ended up with a mish-mosh. That's the technical term for it.

One key fact is that the U.S. has traditionally been a dominant world market for tires. We have a lot of people, and have always had a lot of cars. So, the U.S. Department of Transportation got to set the original nomenclature for tires. That's why, until the 1960s, the wheel size was in inches, the tread width was in inches and there was no sidewall height information (the percentage known as the "aspect ratio"). Back then all tires had the same aspect ratio, which was 90.

But then, technologically superior radial tires were invented in Europe, and the Europeans wanted to sell their tires in the huge U.S. market. And since the only legal requirement for selling tires in the U.S. was that the wheel size be stated in inches (because consumers didn't care back then how wide a tire was), the Europeans just had to change that one number on their tires, and bingo! They had access to the world's largest tire market at the time.

That's when you started seeing radial tires with their widths listed in millimeters, because that's mandated by the Treaty of Versailles. Or maybe it's the Geneva Convention.

Of course, eventually, radial tires were manufactured here, too, and then U.S. tire makers wanted to sell U.S. tires in Europe, so they also adopted the millimeter rating for tread width.

Radial technology also allowed for wider tires and shorter sidewalls. That's when you started seeing aspect ratios on tires. And I'm guessing that, at some point, the U.S. and the U.K. were such dominant car markets that the European manufacturers just said "OK, Uncle!" and started using inches for wheel size in Europe, too. Because if you check out tires sold in Europe, the vast majority have the same nomenclature that we use here.

So, it's really a story of the mashup of globalization. And prepare yourself, John. In 50 years, you'll probably see Chinese characters on the side of your Goodyear.

Get the Car Talk Newsletter

Got a question about your car?

Ask Someone Who Owns One