255 wider than 275?

How are my new 255's (for my car) wider than my Tacoma's 275's? They should be 2 cm narrower, right? I guess manufacturers have some leeway on sizing.

275/70/17 BFG All Terrain
255/35/18 Pilot Sport Cup

If you can't tell, I'm not doing anything very productive at the moment.

 

the tread width of a low profile tire basically matches the section width. tread width on a 70 series all terrain tire will be less than the section width due to having taller side walls.

 

Tread width in mm should be what's on the tire regardless of sidewall height. That is kind of weird. Now I'm curious if my 35s are actually 12.5 wide

 

Tire width number is not necessarily at the tread.

From https://www.goodyearautoservice.com/en-US/tire-basics/tire-size :
the width of the tire measured in millimeters from sidewall to sidewall.

 

This and OP is comparing an unmounted tire to one mounted on a wheel. If the wheel is narrower than the width of the tire (ex. 8" wide wheel with 11" wide tire) which most are, then the mounted tire will not measure it's true width.

 

How wide is the car wheel vs the truck wheel? Wider wheels make tires wider also

 

Tire width is measure on a specified recommended wheel width and is measured sidewall to sidewall it is not tread width.