The first number in a tire size represents the width of the tire in millimeters. In this case, 245 is the width of the tire, while 40 is the width of the tire in the size you're asking about. Since 245 is more than 40, it means the 245 tire is wider.
The second number in a tire size represents the aspect ratio, which is the ratio of the height of the tire to the width. In this case, 45 is the aspect ratio for the 245 tire, while 40 is the aspect ratio for the tire you're asking about. Since 45 is more than 40, it means the 245 tire has a higher profile.
The third number in a tire size represents the diameter of the wheel in inches. In this case, 18 is the diameter of the wheel for the 245 tire, while the tire you're asking about may have a different diameter. If the tire you're asking about has a different diameter, then the 245 tire will not fit on your wheel.
Therefore, based on the information given, the answer is no, the 245 45 18 tire cannot directly replace the 40 tire without considering other factors such as wheel diameter and vehicle-specific fitment requirements.
