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human
ELI5: Why are US tire sizes half metric and half English?

They have the tread width in millimeters but the rim size in inches for most tires except for large off road or other specialized tires.  How did that happen?
gpt
Lets take an example tire size

# ***P215/70R16***

***P*** here designates that the tire is a **P**assenger tire

Other tires may have

T: Temporary

LT: Light Truck

C: Commercial

and 

ST: Special Trailer

*215* here designates the widest point of the outside of the tire, from sidewall to sidewall.  It means that it is exactly 215 mm, which is easier than saying 8.4546 inches. This is also important for precision in installing the tire. the difference between a 215mm tire and a 220mm tire could result in flats or other vehiclular damage if driven for a sustained period.

/70

This is the most confusing part because it means that the tire height (from rim to tread) is equal to 70% of the tires Width from sidewall to sidewall. In this example, the sidewall is 215mm, so the tire height will be 70% of 215, or  150mm

*R*

the R indicated that the tire is a Radial tire. Radials are made without  belts in the rubber to prevent tire punctures. Other types of tires include 

D: Biased Ply, which uses cloth belts and

B:  Belted which use steel belts


*16*

This last bit indicates that this wheel will fit on any 16inch wheel hub. It doesnt need to be in Millimeters, because the wheels are already made to the same spec as the tires.