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Off-roading tires may use a different measurement scheme: Tread width × Outside diameter, followed by wheel size (all in inches) – for example 31×10.50R15 (787 mm × 267 mm R380 in metric designation). The size of the wheel, however, is denoted as 8.5 in × 20.0 in (220 mm × 510 mm).
It is designed to make tire sizing consistent and clear. It replaces overlapping informal systems that ambiguously distinguished between sizes. For example, at least 6 different "26 inch" sizes exist (just by American notation), and "27 inch" wheels have a larger diameter than American "28 inch" (French "700C") wheels.
Prior to 1964, tires were all made to a 90% aspect ratio. Tire size was specified as the tire width in inches and the diameter in inches – for example, 6.50-15. [24] From 1965 to the early 1970s, tires were made to an 80% aspect ratio. Tire size was again specified by width in inches and diameter in inches.
Plus sizing is the practice of replacing an automotive wheel with one of a larger diameter fitted with a new tire of lower aspect ratio so that the new tire has close to the same diameter and circumference as the original tire to minimize any changes in speedometer accuracy, torque and traction control, while reducing sidewall flex and (generally) increasing cornering ability.
ETRTO sizes apply to rims and tyres for vehicles of all types, including bicycles. The great advantage of ETRTO sizing is that it is unambiguous; previously, nominal dimensions were used which were interpreted in different ways by different countries and manufacturers - a problem for the end user.
These range from 16-inch diameter (e.g. Brompton) through 20 inches (e.g. Bike Friday) up to even 26 inches. Wheel rims also come in a variety of widths to provide optimum performance for different uses. High performance road racing rims are narrow, 18 mm or so. Wider touring or durable off-road tires require rims of 24 mm wide or more. [42]
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