255/55R18 designates a tire 255 mm (10.0") wide with a sidewall 55% as tall as it is wide — about 140 mm (5.5") — mounted on a 18-inch wheel. That works out to an overall diameter of roughly 29.0", a circumference of about 91.2", and approximately 694 revolutions per mile. Exact figures vary slightly by manufacturer and tread design.
Of the 94 255/55R18 tire models we track, the most common category is winter (20 models), followed by touring all-season (17) — a good indicator of the kind of vehicles and driving this size is designed for.