285/45R20 designates a tire 285 mm (11.2") wide with a sidewall 45% as tall as it is wide — about 128 mm (5.0") — mounted on a 20-inch wheel. That works out to an overall diameter of roughly 30.1", a circumference of about 94.6", and approximately 670 revolutions per mile. Exact figures vary slightly by manufacturer and tread design.
Of the 30 285/45R20 tire models we track, the most common category is winter (8 models), followed by touring all-season (7) — a good indicator of the kind of vehicles and driving this size is designed for.