# Understanding Tire Sizes for Cars, Trucks & Bikes

February 6, 2013
Posted by

I just got a flat tire and I need to replace it with a new one. What size tire do I need to get? This can be a tricky question, and many of us would just take our cars into a dealership or repair shop and let them deal with it. But it’s not as hard as you might think, and now Wolfram|Alpha provides tools to help you understand tire sizes.

In today’s world, most tires are fairly standardized—this includes car tires, truck tires, bicycle tires, motorcycle tires, and so on. If you look on the sidewall of a tire, you will see some cryptic combinations of letters and numbers. These letters and numbers tell you information about the various size specifications of the tire. For example, you might see something like 215/65R17.

As you can see from the query’s output, those letters and numbers encode the width, aspect ratio, rim diameter, and construction of the tire. Given this information, we can decode it, perform unit conversions, and determine total diameter, circumference, revolutions per mile, and the sidewall height.

The notation used above, known as metric notation, encodes the width of the tire in millimeters, the aspect ratio of the tire as a percent of the width, the tire construction (radial in this case), and the wheel diameter in inches. Another way of expressing the same information on some tires is to use standard notation, which is included in the output of a tire query like 215/65R17. If you want to explore these tire sizes without a real tire size to enter, you can try just asking for “tire size.” The input fields will guide you in constructing your own tire. As with most engineered items, the actual tire measurements may vary a bit, but the nominal values provided by Wolfram|Alpha should be relatively close to that tire’s actual measurements.

You can push things a bit further by including additional optional information. Many tires will also encode maximum speed ratings, load information, and more. For example, how about “LT215/65R17 E 98T M+S.”

Besides car and light truck tires, we have also added support for many bicycle tire sizes. Bicycle tires are a fair bit messier than car tires. The numbers printed on the tire may not give you the actual numbers due to marketing dishonesty and history. You can specify the size of a bicycle tire by using the ISO standard notation, such as “18-622,” which tells you the nominal width and bead seat diameter (not outer diameter) of the tire. Or you can use one of a number of other notations supported in the marketplace. Many of these names may not refer to a single unique size, like “650x22C,” but could refer to a number of different sizes, like a “28×1 1/2,” or a “700C.” For bicycle tires, the sure way to know how big your tires are is to get out a pair of calipers or be prepared to look up data from the manufacturer.

Most motorcycle tires follow the same patterns as car and light trucks, such as “180/55ZR17 M/C” or “MT90SR16.”

With this information, you should now be able to walk into any tire store and actually understand what you’re looking for. Tire jack not included.

1 Comment

are car tire sizes LIKE motorcycle tire sizes? is 170 / 70 / R15 same on both motorcycle and car? OR do motorcycle tire sizes NOT the same as cars?

Posted by Frank August 21, 2013 at 7:29 pm