I agree with you. The first figure is the tyre width. The second figure is the percentage of the width that the tyre wall height is. So 205/55 would give a tyre wall height of 55% X 205 mm = 112.75 mm. The second figure has nothing to do with the width of the tyre. John
Both figures affect tire width. And height. And overall size. From Discount Tire's web site: Section Width - "205" A metric tire's section width is measured in millimeters. This measurement is taken from sidewall to sidewall. In this example, the section width of the tire is 205mm. Aspect Ratio - "65" This number refers to the height of the sidewall. It is a percentage of the section width. In this example, 65 percent of the section width of 205mm equals 133.25. As you can see, increasing or decreasing either figure will have an effect on the tire width. And height too, of course. The first changes directly, larger than 205 makes both height and width larger. The second does both. Increasing the aspect ratio (i.e. to 75) decreases width while increasing height (for the same section width). Decreasing aspect (go to 55 for instance) increases width while decreasing height. That's why you can keep the height (or the width) the same or close while changing the other by adjusting BOTH figures in opposite directions. EDIT: I am off on the way the 205 is measured. It is straight across from inside to outside on the widest part of the profile, NOT around the entire profile from bead to bead. So while the height will vary as described above, the width will NOT. I read my own quote and discerned differently from what Discount Tire clearly said. This getting old is not for the young. Disregard contrary remarks made by me following this, please. My goal is the same as others, to improve our understanding, not confuse.
I do not see anything in your post to show the aspect ratio figure affects the width of the tyre. It only affects the side wall height of the tyre not the width. It does affect the diameter or TPM (turns per mile) of the tyre. John
Gosh this stuff is tough. The 195 is not the width OR the height. It is both added together. Technically it is the width plus the height TWICE. It is measured from the rim edge, down to the ground, across the tire width, and back up the other side until you hit the other rim. Think of the tire and wheel as being cut in half from top to bottom. We're looking at the cut edge from the front or rear of the vehicle, a cross-section in engineering terms. Literally 205 millimeters, the entire distance. The 65 (aspect ratio) merely indicates as a percentage figure how much of the 205 is spent going vertical (height) and how much is spent travelling horizontal (width). The larger that number, the taller (and narrower) the tire. A 75 is tall and skinny, a 45, very wide and short - - typical of today's tires in the monster sizes like 20 inches. (I hate them, personally - - they ride like feces.) I hope that this painful amount of detail is increasing, rather than reducing, comprehension. By the way, I know John to be one of the more informed contributors to this site and hope that my persistence not be taken as harshness or rudeness.
The section width is the widest points of the tyre when mounted on a "specific width wheel rim" as this affects the width measurement. Although I am not in the US I have chosen a US company the details this in more depth that I suggest you read. "Tyre Rack" John.
That is first time I've heard of that. I've always understood the first number (205, in your example) was the section width of the tire horizontally from sidewall to sidewall. The second number (65, in your example) is section height of the sidewall expressed as a percentage of the width. So 205 mm is the width and the height is 133.24 mm (i. e. 65% of 205). In inches that would be 8.1" wide and 5.25" high. I can assure you that my 195/55 tyre measures 410 mm (16"), not 195 mm (7.7") when measured rim edge to rim edge as you describe. TBH, the way you describe it, would make it a pretty convoluted way to go about things. If you are still in doubt here is the Tire Rack Tire Tech that Britprius mentioned.
Ever been absolutely convinced that you're right, God is on your side, and so on? And then... Anyway, which of the following tires has a width of 195: 195/65R15 195/45R16 195/70R17 Correct answer is...: all of them.
