If anyone has found the coding used in area and terminal forecasts to be somewhat arcane, you haven't seen anything until you start investigating tire and wheel codes. It's an amazing and confusing mishmash of inches, metric, percentage-based aspect ratios, (just with tires . . . wheels are a world unto themselves) rules and standards overseen by regulatory agencies which establish their designations according to whether it is in the US, Canada, the UK, Europe or Japan. I have no idea what they do in India or Russia and don't want to know just now. I know, of course . . . it's so elementary . . . that the first number in a tire size is its width in millimeters. Not the tread width (I bet you thought it was tread width) but rather it's the width of the tire at its widest point - somewhere between the outermost tread and the bead of the tire. In millimeters. Is that measurement taken with the tire inflated or not? I don't know and so far can't find out. The next number of course designates the width of the sidewall expressed as a percentage of the first number, the result also in millimeters since the first number is millimeters. That is the tire's "aspect ratio". And the third number is of course the size in inches not millimeters of the wheel the tire is supposed to go on. There are many other letters and numbers that get thrown into the mix but keeping things simple a 235/85/16 tire has a maximum width of 235 millimeters. The sidewall is 85% of 235 or 199.75 millimeters in width and the wheel size is 16 inches. If I want to know the diameter of the tire itself, I'll have to measure it since that information isn't in the code and can't be derived from the code. Which brings me to the Cheng Shins advertised by Aircraft Spruce mentioned in earlier posts. Specifically the ones designated 11/4.0/5. What do those numbers mean in terms of the familiar tire codes we use for our cars and trucks? Are all these numbers in inches? Jumping to conclusions here for I really don't know, I would guess the 11 means the tire is 11 inches in diameter, but what is the 4.0? If 4.0 is the width of the sidewall in inches, then the tire would have a diameter of 13 inches, not 11 inches. And how does the 11/4.0/5 designated tire relate to a tire that is designated 4.10/3.50/5? My Cheng Shin tire with the latter designated codes are 13 inches in diameter uninflated but the only reason I know that is because I measured it. Going by the code one would think the diameter would be 3.50 + 3.50 + 5, or 12 inches in diameter. But they aren't. They're 13" in diameter. If anyone understands these codes and aren't just guessing, please advise. There must be some point as tires grow smaller where our familiar mishmash of millimeters and inches used to code car and truck tires goes to another system . . . actually more than just one system . . . at least two, as illustrated in the examples above. How do these systems work? In neither case does my guesswork at what the codes mean correspond to the actual dimensions of the tires. Thanks, Mike KSEE
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