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Tire sizes explained

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  • Tire sizes explained

    Tires like alphabet soup. They like it so much that all of them have some printed on the side of them: P195/60R15, P175/70R14, LT235/60R17... This alphabet soup tells a great deal about the design and dimensions of any given tire, if you know how to read it. That's what this article is about.

    Break it down

    You can break any of these tire sizes up into its component parts. We'll use the stock tire size of an SC2 as an example.

    P 195 / 60 R 15

    "P" is the type of tire.
    "195" is the tire's width in millimeters.
    "60" is the tire's aspect ratio.
    "15" is the size of the wheel it's designed to be mounted on.

    Tire type

    "P" means that this tire is designed for passenger cars. Any tire you buy for a Saturn, no matter what size, will begin with a P. The only possible exception would be the VUE, which, being an SUV, may have LT (light truck) tires available for it. But given that the VUE is designed more for on-road use than off, they'll usually have P tires, too. LT tires are more for pickup trucks and such that actually see some use other than driving to work or the grocery store.

    Tire width

    A 195 tire should be 195mm wide. I say "should" because in real life, there's actually some variation from manufacturer to manufacturer. In the racing world, it's fairly well known that a 205 Hoosier tire will be wider than a 205 Kumho tire, even though both are 205s. But generally speaking, particularly for street tires, any variation will be minimal. And there is a definite visible difference between a 195 and a 205.

    Tire aspect ratio

    This is where it gets a little more tricky. A tire with a 60 aspect ratio will be 60% as tall as it is wide. In other words, a P195/60R15 tire will be 195mm wide and 117mm tall (60% of 195). A P195/50R15 would be 195mm wide and 97.5mm tall - a lower profile tire.

    But keep in mind that this is a percentage. If you upgrade from a stock P195/60R15 to a wider P205/60R15, the height will still be 60% of its width - 60% of 205 rather than 195, or 123mm tall - 6mm taller than stock. While wider tires have obvious traction advantages, if you're also going for a low profile look, don't just go for a wider tire - it'll also be taller, NOT shorter!

    But if you switched to a P205/55R15, it would be just as wide, but only 55% as tall - 112.75mm, to be exact. That's about 4mm shorter than stock.

    Wheel size

    The SC2 comes with 15" wheels, so the tires that come with it are designed to be mounted on 15" wheels - hence the "15" at the end. It doesn't get much simpler than that. But it improves performance, too. A shorter sidewall means there's less sidewall flex, improving handling and the car's reaction to your control inputs. The tire is smaller than stock, which effectively lowers your gearing - bad for maximum speed, but better for acceleration. Finally, a shorter tire causes the car to sit lower to the ground.

    Many Saturn autocrossers run P205/50R15 racing tires, which are wider and significantly shorter than stock. They grip better, respond better, and an SC2 rides half an inch lower than on stock size tires - a reduction in ride height that's even legal in stock class! (Any size tire is allowed as long as it fits on an OEM sized wheel.)

    There are also downsides. Road noise, for instance. Fuel economy won't be as good, since a shorter tire has to spin more rapidly than a stock tire at the same speed. This also means your speedometer will read too high. And since tires also help absorb bumps in the road, a shorter tire acts like a shorter spring, and can't soak up the bumps as well.

    The key is to choose the right tire for the right job. For racing, the downsides are irrelevant and going fast is the important thing. For a modified daily driver, you might compromise a bit. I run P205/55R15 summer tires, a bit wider than stock but not too much shorter. And for winter, a narrower tire will cut through snow and ice to the pavement better than a wide tire, which tends to float, so my snow tires are the stock size.

    Tire size calculator

    There's an excellent tire size calculator tool on (It works for any car, not just Miatas. :twisted:


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