The 65 represents the ratio of sidewall height to the width of the tire. So if you have 255/65 it means the sidewall height is 0.65 x 255mm which is 190mm. Add this to the rim size of 15 or 16 inches (why can't we just use metric?) and get the circumference from 2 x 22/7 x radius. Also, these measurements aren't exact. Different tires with the same "size" will have different measurements.