The old inch-measurement tires follow the pattern explained above- - - -tread width in inches is the first number and rim size is the second. Later model metric-dimension radial tires get a little more specific- - - -Tread width in millimeters, then "aspect ratio", and then rim diameter- - - -usually still in inches for some reason! For instance: 225/75R15 would translate to 225 MM of tread contact with the road, a 75% "aspect ratio" which means the sidewall from the rim to the tread is 75% of the tread width, on a 15" rim. The "R" stands for "radial". As the aspect ratio getc smaller, the height of the tire from the rim to the ground decreases, so an 85, a 75, a 70 and a 60 series tire will get progressively shortet (and ususlly wider) as the heoight of the sidewall gets shorter. Some radials for vintage trucks run very tall sidewalls in relation to tread wifth, but that's an exception to the usual practice.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!