Tire Size Chart: Find the Right Size

Reading a tire size chart can be confusing, whether you're actually intending to replace your tires soon, or just casually checking prices. The following information will help you read these charts correctly and identify the proper size for your vehicle. 

To find the correct tire size for your vehicle, first look closely on the sidewall of one of the front tires. On the tire sidewall (the part of the tire that you can actually wash), look for a sequence of letters and numbers in the pattern of the following tire size measurements: 185/60R-14, 225/50R-16 or 255/40ZR-18 for example. Don't worry if your tires don't match one of those 3 size examples; just jot down the exact size listed on the sidewall of your front tire. You should also check one of the rear-tire sidewalls, in case the two rear tires need to be a bit larger than the two front tires. Your owner's manual should help you confirm the size(s) that you have written down. 

For example, let's imagine that you've written down the following tire size directly from the sidewall: 225/50R-16. The first part of this measurement describes the width of the tire in millimeters; in this example, the tire is 225 millimeters, or roughly 8.9 inches, wide (225mm/25.4mm per inch = 8.86 inches). 

Now that the tire width has been identified as 225mm, we should consider the next part of the data in our example: 50R. Specifically, the numerical portion (50) tells us that the tire sidewall height (or aspect ratio) is 50% of the tire's width; in this example, the "height" of the tire is 112.5mm (or 4.43 inches). Finally, the letter R indicates that our example is not designated as an ultra-high speed tire; if it were so designated, it would have carried a "ZR" rating (for example, 225/50ZR-16). Only tires rated to cruise at speeds above 150mph can earn the ZR rating. Other speed ratings are designated by other letters.  

The last part of the tire-size rating indicates the wheel-size matchup. From our example, we can quickly determine that the tires will match up with wheels that are 16 inches in diameter. 

As listed on a tire comparison chart, each dimension of the tire (width, height, speed rating and wheel size matchup) is important, because it corresponds to key dimensional and safety data for a particular vehicle. For example, if the tires are too wide or too high, they may rub against the body/chassis of the car while turning or when going over bumps. Moreover, an incorrect tire size can confuse the wheel speed sensors on the car, which could lead to anti-lock braking system (ABS) trouble. Furthermore, if your vehicle has 16-inch wheels, and you've ordered tires for 18-inch wheels by mistake, then you will simply have to send them back.

Thus, it's best to take your time, to identify the exact size of the tires on your vehicle, and to confirm the original manufacturer's recommended tire size (with your owner's manual or a well-informed mechanic), before scanning a tire size chart and placing an order.

Related Questions and Answers

The tire manufacturer has spent a lot of time finding the recommended tire size for your vehicle, so it does pay to drive on tires of the correct size. The interesting point here is what some engineers call the "slop" factor. It is a plus/minus factor, meaning that within the range of the "slop" factor, everything will be all right. Since the "slop" factor on cars is about 3 percent, it means that you can drive on wheels that are three percent larger or smaller than those recommended by the owner's manual, and you will have no problems.

Your owner's manual gives you lots of information on tire width and size when you need to calculate the tire size. All you need to know is your car's current wheel/tire. Let's say your tire is 225/45R17. This means your tire is 225 mm wide. Now, you have to add in the impact of the aspect ratio - the ratio of sidewall height to width. You already know it is 0.45, so, using the figures, multiply 225 mm X 0.45. This equals 101.3 mm, which is the distance from the wheel to the top of the side wall. Since there are two sides to the tire, one top and the other bottom, you multiply the 101.3 X2, which equals 202.6. To this point, you will know the size of the tire is 225+202.6, but now the rest of the figure is in inches. So we have to translate everything to inches by dividing 225+202.6 by 25.4 (8.86+7.98)+17=33.84. The wheel is 33.84 inches. From looking in the owner's manual, you will know that the wheel will accommodate anything up to a 40 inch tires. So given that, you can increase the size of the tire by three inches (17 to 20).

While your vehicle tire size does affect the cars ability to turn, especially if the tires are too big for the wheel wells, ultimately vehicle tire size will not affect the vehicle's turning radius. The turning radius is set at the factory and made for a specifically sized tire. It is also put into the owner's literature in this manner. So while you may have a set of oversized tires on your wheels, the only thing you'll have trouble doing is turning your car. The radius remains the same. The way to restore this is to ensure you have properly sized wheels and tires for your car, and then your turning radius issue will be gone.

Attempting to drive a car with two different tire sizes is the same as rafting down the Colorado River without paddles. Not only do the different size characteristics compromise the ride quality, they also make good handling nearly impossible, as it feels as if you are driving a three-wheeled car with flat tires. In other words, it just doesn't work. The only solution to this problem is to purchase four properly sized tires and have them installed and balanced on your vehicle. You'll notice a great increase in your car's ride and handling. Additionally, it is much safer, as well.