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How Do I Safely Upsize the Wheels On My Car?

Oversized TiresWith low profile tires being all the rage, moving to a bigger wheel is an easy way to update the look of your car. It can also improve performance: A shorter sidewall flexes less while the vehicle is making turns, thus maintaining grip, while a wider tire improves lateral stability. Switching to a bigger wheel can open up a wider selection of compatible tires, particularly performance models.

However, there are some tradeoffs: Tires are lighter than wheels, so as wheel size goes up, so does weight. This negatively impacts fuel economy and acceleration and makes the steering heavier. The shorter sidewall also provides less cushion, giving the car a harsher ride.

 

How big can I go?

If the new wheels and tires are bigger than the originals, the tires may rub against the fender well when making tight turns or when the suspension bottoms out. Speedometers calculate speed by determining the distance traveled for each wheel rotation, so a size change can make the reading inaccurate. To keep the suspension and speedometer functioning correctly, both the stock diameter and width of the wheels and tires needs to be maintained.

As a general rule of thumb, it’s safe to fit a tire up to 20 millimeters wider than stock on the original rim. The actual width of the tire will vary depending on the width of the rim: The tire will expand 5 millimeters for every half inch (12.5 millimeters) increase in rim width.

Moving to a different rim gets a little more complicated because tire sizes are a combination of metric and percentage measurements, while wheel sizes are in Imperial measurements.

For example, the car currently has 225/45R15 tires. Here’s what that means:

  • 225 – Tire width in millimeters
  • 45 – Sidewall height as a percentage of tire width
  • 15 – Rim diameter in inches

To convert the wheel size into millimeters, multiply by 25.4:

  • 15 inches x 25.4 = 381 millimeters

Next, calculate the sidewall height by multiplying the tire width by the height percentage:

  • 225 millimeters x 0.45 = 101.25 millimeters

Add the two numbers together, to get the total height of the wheel and tire:

  • 381 + 101.25 = 482.25 millimeters

To keep speedometer error in line, the new tire and wheel should be within 3 percent of the original combination’s height. Moving to a 16 inch (406.4 millimeters) rim would require a tire with a height of 75.85 millimeter, 34 percent of the 220 millimeter width, or 220/34R16. The closest size manufactured is 220/30R16, which is well within the 3 percent size margin.

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