# Why are car tire sizes in both metric and imperial units?

**URL:** <https://boards.straightdope.com/t/why-are-car-tire-sizes-in-both-metric-and-imperial-units/398798>\
**Category:** Factual Questions\
**Created:** [April 5, 2007, 12:53am UTC](https://boards.straightdope.com/t/why-are-car-tire-sizes-in-both-metric-and-imperial-units/398798 "2007-04-05T00:53:56Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![King\_Friday](https://avatars.discourse-cdn.com/v4/letter/k/43a26b/32.png) [@King\_Friday](https://boards.straightdope.com/u/King_Friday)\
**Post date:** [April 5, 2007, 12:53am UTC](https://boards.straightdope.com/t/why-are-car-tire-sizes-in-both-metric-and-imperial-units/398798/1 "2007-04-05T00:53:56Z")

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When you size up a car tire, you use something like 195/65/15. 195 millimeters, 65% (of 195), and 15 inches. Why is it so mixed up like that? Why can’t the diameter be measured in cemtimeters, or the tread be measured in inches?

The 195/65/15 could easily be 7.7/65/15, or even 195/65/38.

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**Author:** ![MrFloppy](https://avatars.discourse-cdn.com/v4/letter/m/43a26b/32.png) [@MrFloppy](https://boards.straightdope.com/u/MrFloppy)\
**Post date:** [April 5, 2007, 1:34am UTC](https://boards.straightdope.com/t/why-are-car-tire-sizes-in-both-metric-and-imperial-units/398798/2 "2007-04-05T01:34:08Z")

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My WAG would be that wheels are made in whole increments of 1" with the odd 1/2" size squeezed in there.

Tire widths generally are in 5mm or 10mm increments.

What confuses matter more is that some larger tires are all imperial. FOr example, a popular size is 31x10.5R15 (31" overall diameter, 10.5" width and 15" rim dia.).

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**Author:** ![Gatopescado](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gatopescado/32/5711_2.png) [@Gatopescado](https://boards.straightdope.com/u/Gatopescado)\
**Post date:** [April 5, 2007, 4:40am UTC](https://boards.straightdope.com/t/why-are-car-tire-sizes-in-both-metric-and-imperial-units/398798/3 "2007-04-05T04:40:47Z")

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My God! Why haven’t I ever asked this question? I’m kind of a tire fiend.

Bravo!

(My best guess: They didn’t always. Tires/wheels used to be pretty skinny and there weren’t that many sizes out there. As “modern” times rolled- no pun intended- around, the need for a better system emerged. Someone smarter, and who drinks less, will come along and clear this all up, I’m sure.)