What is the difference between 255 70r18 tires and 275 65r18 tires?

The difference between 255/70R18 and 275/65R18 tires primarily lies in their width and aspect ratio. The first number represents the tire width in millimeters, so the 275 tire is wider than the 255 tire. The second number indicates the aspect ratio, which is the height of the tire's sidewall as a percentage of its width; the 70 aspect ratio means the sidewall height is 70% of 255 mm, while the 65 aspect ratio means the sidewall height is 65% of 275 mm. This results in the 275/65R18 tire being slightly wider but having a shorter sidewall compared to the 255/70R18 tire.