I purchased this size tire
"I purchased this size tire (T155/70R18 112M) as a spare for my 2011 Mustang GT (standard brakes), because the original spare - Maxxis T185/55R18 109M - was no longer made, was 16 years old and showing signs of sidewall breakdown. This brand and size tire was used on multiple years after my Mustang. It fits perfectly on the 18 inch rim. The diameter is approximately 26. 5 inches, which fits in the wheel well of the trunk. While it is narrower than the original, the new width is consistent with other compact tires used today. It even has a higher load rating than the original and it appears to be a perfect replacement for the original tire. As a spare, I will not be able to evaluate traction, performance, comfort or treadlife at this time."