How to Change Tire Sizes Like a Pro

Summer tires on a car.
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There are good reasons for changing the tire size on your car. Enthusiasts do it to improve the looks and performance of their vehicles. Other motorists do it to save money or because they drive under challenging weather conditions. Whether you're purchasing larger tires and rims or smaller ones, here's what you need to know about sizing.

Upsizing

Replacing the factory-standard wheels with larger tires and rims is called upsizing or plus sizing.

It is most often done for looks and performance benefits. Larger wheels are undeniably striking, and there is no more effective way to change the look of a car than by putting larger wheels on it. 

According to Car and Driver, putting on larger wheels up to 18 inches will positively affect cornering, grip, braking performance, ride comfort, and steering feel, while negatively impacting acceleration and fuel economy due to the greater weight of larger wheels. At 19 inches and beyond, the testers found that the positive effects started to go away, while acceleration and fuel economy got worse.

Downsizing

Downsizing is the opposite of plus sizing; you're installing smaller wheels. Car owners typically do this if they own a second set of tires, for example, snow tires they install every winter. Snow tires tend to become much more expensive at sizes larger than 17 inches. In addition, the narrower the tire, the more effective it tends to be on snow and ice.

So if you have 18- or 19-inch wheels and want an extra set of wheels for snow tires, it could be a good idea to go down to 17- or 16-inch wheels.

Diameter Is the Key

Your speedometer, odometer, traction control, torque, and gearing settings are all based on the distance that your tire travels over one complete revolution, which is determined by the outside diameter of the tire-and-wheel assembly.

A tire with a different outside diameter travels a different distance over that one revolution with a different amount of torque. When changing the diameter of your rims, you must make sure that the new assembly keeps the same overall diameter as the old or your speedometer and your traction control settings will be off. 

How to Size Your Tires

Tires are sized using a three-number code, such as 225/55/16. For a tire of this size, the first figure (225) represents the width of the tire in millimeters. The second figure (55) represents the ratio of the width to the height; that is, the aspect ratio is 55 percent of the width, or 123.75 mm. The final number (16) refers to the inner diameter.

The outside diameter of the tire, also known as the standing height, is determined by how much sidewall it has, called the aspect height. In order to keep the same outside diameter when you gain an inch of rim size, you must lose an inch in the standing height of the tire, and vice versa. To determine the proper size requires a bit of math.

To get the standing height of the tire, one must multiply the aspect height by 2 (for the top and bottom sidewalls) and add the 16-inch inner diameter of the tire.

After converting from millimeters to inches, this yields a standing height of approximately 25.74 inches. Once you have the standing height of the old tire, you must then match it on the new tire:

  • Multiply the width by the aspect ratio expressed as a decimal (225 x 0.55 = 123.75).
  • Convert the aspect height to inches (123.75 / 25.4 = 4.87).
  • Multiply the aspect height by 2 (4.87 x 2 = 9.74).
  • Add the inside diameter of the tire (16 + 9.74 = 25.74).
  • Repeat for the size of the new tire.

Don't worry if math isn't your strong suit. You can find plenty of tire-size calculator apps and websites that will help you get the right fit every time.

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Phillips, Sean. "How to Change Tire Sizes Like a Pro." ThoughtCo, Sep. 25, 2017, thoughtco.com/plus-and-minus-sizing-your-tires-3234471. Phillips, Sean. (2017, September 25). How to Change Tire Sizes Like a Pro. Retrieved from https://www.thoughtco.com/plus-and-minus-sizing-your-tires-3234471 Phillips, Sean. "How to Change Tire Sizes Like a Pro." ThoughtCo. https://www.thoughtco.com/plus-and-minus-sizing-your-tires-3234471 (accessed November 23, 2017).