255/50R18 designates a tire 255 mm (10.0") wide with a sidewall 50% as tall as it is wide — about 128 mm (5.0") — mounted on a 18-inch wheel. That works out to an overall diameter of roughly 28.0", a circumference of about 88.1", and approximately 719 revolutions per mile. Exact figures vary slightly by manufacturer and tread design.
Of the 3 255/50R18 tire models we track, the most common category is grand touring all-season (1 model), followed by ultra high performance summer (1) — a good indicator of the kind of vehicles and driving this size is designed for.