Tires: sizes, rim width, zr20

Question
is 245 30 zr20 and  255 30 zr20 the same size tire

Answer
Jay,

No, they are different sizes.  The 255/30R20 is wider by 10 mm, taller by 6 mm, has about 100 # more load carrying capacity, and requires a minimum of an 8 1/2" rim  and a maximum of a 10. (Compared to the 245/30R20, where the rim width range is 8 to 9 1/2.)

However, the physical dimensions will vary somewhat and sometimes it may appear that these are physically the same size.