3 ms·

I noticed that the other day too! Dumb question but what does the rest of the world do? Do they convert inches to mm or is the inches diameter just an approxima

• analog31 8y ago
  I can't speak for cars, but there's a similar situation with bicycle wheels and tires. There are all sorts of historical measures, such as "26 inch" and "700c," but at the same time most rims and tires are marked with ISO dimensions that are in mm and quite well defined. So for instance my "27 x 1-1/8" tires are also ISO 28-630, where 28 is the approximate inflated width, and 630 is a particular dimension of the rim. When I buy a new tire, I look at the ISO number to make sure I got the right size. I recently restored an old Schwinn with its own oddball tire diameter, but ISO numbers published by modern tire makers got me up and running with new tires. I think that in a lot of cases, English vs metric is a moot point because a tool is so specialized (spark plug wrench) or because production tools are computerized and switch unit systems as needed. On a milling machine, all dimensions boil down to the number of stripes on a precision glass scale, regardless of what units you choose to specify. Historically, there are bike parts with mm diameter and English thread pitch, such as some old French pedals.