What is the dia of a 165-15 tire?

The diameter of a 165/15 tire can be calculated using the tire's aspect ratio and rim size. In this case, the tire width is 165 mm, and it fits a 15-inch rim. The aspect ratio is typically assumed to be 80% for a standard tire unless specified otherwise, which gives a sidewall height of approximately 132 mm (0.80 x 165). Adding the sidewall height twice (for both sides of the tire) to the rim diameter (15 inches or about 381 mm) results in a total diameter of approximately 645 mm, or about 25.4 inches.