This info was provided to me courtesy of Sukoshi Fahey at Avon Tyres
Sizing is confusing as there are 5 methods in use. There are three technical organizations that all manufacturers adhere to. The organizations that set the parameters/ranges for permissible ranges for all sizes are the European Tire & Rim Technical Organization, the Japanese Association of Tire Manufacturers and the US Tire & Rim Association.
The 3 most common ones are inch sizing, Alpha numeric and Metric. Metric sizing provides the most information, for example a 140/90H16 where the approximate width is 140 mm, the key word being approximate. Bear in mind that manufacturers are allowed approximately a 5% variance from standard. This is why no two tire measurements from manufacturers are the same. They may both marked 140 but one width may be 134 mm and another 145 mm.
And the size ranges can overlap. One manufacturer’s “140” can actually be narrower than another manufacturer’s “130”. It’s important to check what the actual width of a replacement tire is with the manufacturer to determine if it will fit. Most tire manufacturers’ literature will include overall widths in inches and/or mm and the rim size that each width is measured on. Take a size and check one manufacturer against another and you’ll be able to see quite easily that there can be as much as a quarter to a half inch difference in width between the same sized tires. Be aware that this can cause fitment problems!
Okay, so now we go back to the 140/90H16 tire. The second set of digits, the “90” is the aspect ratio of the tire. The aspect ratio is the relationship of the height of the sidewall to the width of the tire. The aspect ratio of a tire is always expressed as a percentage of height to width, so the “90” in a 140/90-16 means that this tire is 90% as high as it is wide or 126mm tall.
As a 90% aspect ratio or lower tire reaches its maximum adhesion around a corner it will begin to slip. You will start to feel that it’s slipping, but it will fall off on a relatively gradual basis.
On the other hand, lower aspect ratio tires have comparatively better grip, but when they reach their maximum adhesion, they drop off, or begin to slip, more rapidly. There is less warning.
So, low aspect ratio tires are better suited for the skilled rider. And when I say skilled rider, I don’t just mean someone who has been riding for 10 or 15 years. I’m referring to an accomplished rider, who knows how to handle a high performance motorcycle and will be able to sense when he is reaching the maximum limits of his abilities and the machine’s.
Again, let’s go back to the 140/90H16. The letter “H” following the aspect ratio signifies the speed rating of the tire. In the U.S. the Federal Motor Vehicle Department tests only at speeds up to 85 mph. So the tire manufacturers have borrowed the European speed rating system which goes higher. There are five levels in European system: N, S, H, V, W. The N rating is good for sustained speeds up to 93 mph, S is good up to 112 mph, H for speeds up to 130 mph, V 149 mph, W over 149 mph. One common error, regarding speed ratings, is to assume that the higher the speed rating, the better the tire. This is not necessarily true. Application is what determines what tire is best. One problem with this system, however, is that there are no international standards for load and time at these speeds. It’s strictly up to the integrity of the manufacturer of the tire as to how long and at what load, these sustained speeds will be tested.
.Finally, the “16” at the end of the 140/90-16 is the diameter of the wheel on which the tire is to be mounted.