i believe the firs two numbers are in centimeters and then the last number is in inches.
that will be true for most tires unless you run into some older american tires or some tires used for drag racing.
first number is the width of tire in mllimeters. Second number is the the height of sidewall in ratio to width of tire. In the case of 205/45/17, the sidewall would be 45% of the width of the tire, which would be 92.25mm. And you know what 17 is already.