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I'm quite familiar with the code used to define tire sizes, but one aspect of the code always confuses me. The second number defines the sidewall height as a percentage of the tire's width. As an example, my Ford ZX2 has 195/60R15 front tires and 205/60R15 rear tires. In both cases, the sidewall height is 60% that of the tire's width. Does this mean the rear tires have a slightly larger diameter than the front tires?

If the answer to that question is yes, then that is where I am confused. Since choosing a wider tire (particularly for handling reasons, as I've done with my Ford since it's far less squirrely at highway speeds than with 195s) means the tire is also larger, you would have to compensate by choosing a lower sidewall height value. Thus, one variable of the tire's dimension affects two values in the code.

Doesn't this make choosing an appropriate tire less intuitive and more difficult for buyers?

Wouldn't it be more intuitive to express sidewall height in reference to the wheel diameter versus the total wheel+tire diameter?

For example, if the second number referred to the percentage of diameter that is occupied by the wheel, then 60R15 would mean the tire fits on a 15-inch wheel, and the sidewall is 5 inches high for a total diameter of 25 inches (?/15 = 100/60; thus, 15 x 100 / 60 = 25). This matches closely with the actual diameters of my car's wheels.

Under the current code, I couldn't opt for a 150/60R15 or a 300/60R15 and get the same total diameter. I'd have to change that second number as well (higher for the 150mm wide tire, and lower for the 300mm wide tire). But under a percentage-of-diameter variant of the code, 150/60R15, 200/60R15, and 300/60R15 would all have exactly the same sidewall height and fit on exactly the same size wheel—they would just have different tire and tread widths! Or I could simply choose a 200/80R15 instead of 200/60R15 and have a lower profile tire and be done with it. I'd only need to futz with one number instead of two.

Why isn't the sidewall height merely expressed as a percentage ratio between the wheel diameter and the tire diameter?

  • Asking 'why' seems offtopic and irrelevant. It may be legacy, but it doesn't change anything you need to know to choose tyres. – Rory Alsop Dec 6 '14 at 20:52
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You are given yourself undue consternation. The tyre sizing protocol is just one way of identifying a tyre size which has been universally adopted. The tyre size for a vehicle is decided at its design stage. To change to a differant size, say my cars tyres which are 205/55 16, the maths are : Multiply the tyre’s width by the aspect ratio to get aspect height (205 x 0.55 = 112.75), Convert into inches (112.75 / 25.4 = 4.44), Double the aspect height (4.44 x 2 = 8.88), Add to the inside diameter of the tyre (8.88 + 16 = 24.88). 24.88 is the overall diameter of my tyres. Now if I decided to change tyres they must be within 1% of this same calculation result. If not they will cause problems with the speedo and the transmission gearing if I change rim diameter. The popular change would be a wider tyre which does not radically alter this overall diameter but will present the likelyhood of fouling on the bodywork or suspension at the front, unless the vehicle is adapted for the tyre size by offsetting the rims and flaring the wheel arches. One other consideration is the contact area of the tyre and road. At design of the vehicle the contact area can be thought of as a square. Increasing the width of the tyre changes the contact area to an oblong. This oblong area becomes increasingly narrow with increasing width and at some point starts to decrease road holding. (I didnt believe it at first either). Lowering tyre pressures to compensate causes excessive tyre wear and increased fuel consumption.

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