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What Do All Those Numbers on My Tires Mean?

One of the most basic aspects of car care is replacing your tires when they are worn. If you’ve ever shopped for tires online, they will ask for your tires measurements which look something like 195/60R15 All-Season H-Rated. That’s pretty complicated for just a rubber wrapper around your rim! All of that information actually means a lot and can dramatically impact your car’s ride, performance and the lifetime of the tire.

Even though your car came with a certain type/size of tire, there is actually quite a bit of flexibility about what size and type you can put on that stock rim, depending on your needs. Usually manufacturers pick a tire size that is a good “all around” tire for comfort, performance and fuel economy. However, what a Subaru in Colorado needs is not what is best for a Subaru in Florida. Let’s go over what all that info on your tire means.

The first number you see is the width of the tire in millimeters when it’s mounted on the rim, but before it’s on the car. So a 190 is less wide then a 225. Width of a tire increases surface contact, which generally means it will corner better. This is why drag racers have really wide rear tires (to increase the surface contact with the wheels that are powered), and tiny front tires (because it doesn’t need to turn, the thinner tires lower the rolling resistance). Generally sports cars and trucks have wider tires, while smaller economy cars have less wide tires.

The second number you’ll see is the tire’s “aspect ratio”. Here you’ll need to do a little math. If the number is 50, what it’s saying is that the height of the tire, from the rim to the tread, is 50% what the width of the tire is. So if your tire is 200mm wide, with a 50 aspect ratio, then the height of the tire is 100mm. Aspect ratio is generally between 40 and 60 on most cars, and this affects how much tire you see from the side. Generally a smaller aspect ratio is sportier, but a rougher ride, while a larger one is more comfortable and better for fuel economy.

The last part of the measurement, R15 is the radius of the tire, this time in inches! So R15 means the tire is for a 15″ rim. Rim sizes have been increasing greatly over the years, generally a smaller rim means better fuel economy but worse handling. There is a tradeoff between larger rims as well though, so typically manufacturers choose a rim size that is fairly neutral.

Finally, you have a tire that fits your car, and you understand the bizarre combination of measurements that make up tire size (mm)/(math equation)R(inches). Now you need to pick the type of tire. Almost all manufacturers ship with an “All-Season” tire, and for 90% of drivers this is fine. An All-Season tire is for exactly like it says, decent in the snow, decent in the summer, not perfect for either.

If you really want to specialize, or live in a more extreme climate, you can also get summer or winter tires. The main difference is the depth of the tread in these tires. Summer tires are almost completely flat (meaning if you hit snow you are screwed) and Winter tires can chew through some snow (but will be loud and inefficient on a dry road).

The last thing to keep in mind is the rating of the tire. This is an A-Z scale, where the higher the letter, the more likely it is to be able to perform at high speeds. Typically most cars start with S-Rated tires, which are good up to 112mph. If you are planning on going faster then this (and live near the Autobahn?), you’ll want to move up to a higher letter.