I little late now, but Tirerack's Tire Tech has a great explaination.

 

Thanks man you just proved me correct.

 

Glad you are getting it. I had a guy at NTB tell me that a 285/70/R16 and a 285/70/R17 would be the same size tire. Needless to say, I didnt buy tires from him.

 

Correct......... as in you were wrong all this time.

Glad you got it figured out now.

 

Well to correct my first thread, I have always thought the first number coincided with the width. I have always thought that the second coincided with the height, but I didnt exactly know what it stood for. I had coworkers arguing that the first number was the height, and that the second was the width. I did all of this to prove that I was basically more correct than they were. Needless to say, I shut those jackasses up.

 

now that you have figured out metric sizes, want to try for SAE measurements? Those are much easier to understand and are backwards of metric tires.

i.e. 35x12.5R17

The first number is the height from top to bottom(circumference) in inches. The second number is width of the tread in inches. The third number, of course, is the size of the wheel to be mounted on(hole in the center of the tire).

In this case your co-workers were right. I wouldn't tell them though

 

Yeah thats the thing, I allready told them that they were thinking of those measurements before all of this and they still argued. Its all good though, I educated them. Thanks for everything guys.

 

Circumference is the distance around a circle. Diameter describes a circle's (tire's) height or width. The first number is diameter or hight as you describe. Circumfrence would be diameter times the constant Pi.

 

Gotch. I guarantee this is educating more people than we think.

 

The above tire info link says that the first number is the meaurement of the tread. From outer wall to inner wall. Not the diameter of the tire.