Low Profile Tire Plus Sizing for Cars or Trucks

How to Determine Proper Performance Tire Size for Bigger Wheels

Aug 18, 2009 James Clausen

What's the best way to figure out what size low profile tires should be used when changing to a bigger wheel or rim? Learn how to choose low profile tires.

Plus sizing actually refers to the wheels, meaning to go to a larger diameter wheel. Plus sizing a wheel with low profile tires just plain looks better on most cars and trucks.

Another big advantage to plus sizing is in vehicles handling performance. A larger diameter wheel also means a larger width tire. A larger width tire means more rubber contact with the road, which means better handling, especially when cornering.

Advantages to Staying Close to the Original Tire Size Diameter

Most of today’s cars and trucks are computer controlled. Wheel speed plays an important role in sending signals to the vehicles computers, which in turn tells other vehicle components what to do. By going to drastically different tire diameters, some of the vehicle components that may be adversely affected include:

• Speedometer reading accuracy
• Odometer accuracy
• Transmission shift points
• Computerized traction control
• Anti-lock brake system (ABS)

So it’s important to try and stay as close to the original factory tire diameter as possible. Going to a larger wheel and keeping the original tire diameter will still allow for a low profile tire that will improve the appearance and handling of the vehicle.

How to Choose the Right Diameter Tire for Wheel Plus Sizing

The tire industry doesn’t make it easy to tell what the overall diameter of a tire is by looking at the tire size. To even complicate matters, part of the tire size is millimeters (width) and part of the tire size is in inches (wheel diameter). The first thing that needs to be done is to determine the overall diameter of the original tires. As an example lets say the original tire size is P225/60R16.

1. Tire Width - The 225 represents the width of the tire in millimeters. To convert the width into inches, multiply the 225 by .03937, which equals 8.86 inches rounded. So the width of the tire is 8.86 inches.
2. Sidewall Height - The 60 represents the aspect ratio of the sidewall height to the width. In this example the sidewall height is 60% of 225 millimeters or converted 60% of 8.86 inches, which equals 5.32 inches rounded. So the sidewall height of the tire is 5.32 inches.
3. Wheel Diameter – The R just stands for radial, so this is not a concern for tire plus sizing. The 16 however indicates the diameter of the wheel in inches.
4. Overall Tire Diameter- Now that everything is converted to inches, simply double the sidewall height (above and below the wheel) and add the wheel size. So the original overall tire diameter is 5.32 + 5.32 + 16.00 or 26.6 inches rounded.

Plus Sizing to 20 Inch Wheels as an Example

Again the diameter of the new tires should be close to the diameter of the original tires. Using the formula above to find the right tire diameter, it’s best to play around with some sizes. Using 20-inch wheels as an example, some of the tire diameters that are close to the original tire diameter are:

• 245/35R20 - Overall Tire Diameter 26.8” (.03937 X 245) = 9.65” tire width X 35% aspect ratio = 3.38” sidewall height. 3.38 + 3.38 + 20.0 = 26.8” tire diameter
• 235/35R20 - Overall Tire Diameter 26.5” (.03937 X 235) = 9.25” tire width X 35% aspect ratio = 3.24” sidewall height. 3.24 + 3.24 + 20.0 = 26.5” tire diameter
• 275/30R20 - Overall Tire Diameter 26.5” (.03937 X 275) = 10.83” tire width X 30% aspect ratio = ” sidewall height. 3.25 + 3.25 + 20.0 = 26.5” tire diameter

Since the original overall tire diameter is 26.6 inches, all of the above examples are close. If performance is the main reason for plus sizing, the 275/30R20 would be the best choice. The original tire width is 8.86 inches and this choice in tire size would add close to 2 inches on the width of the tire. Be careful to make sure that adding 2 inches to the width of the tire doesn’t interfere with the vehicle fender well when turning.

Always pick a good speed rated tire, especially if performance is the reason for plus sizing. By plus sizing to a 275/30R20 from a 225/60R16, the tire diameter virtually stays the same but 2 inches are added to the width. The sidewall height also dropped by over 2 inches from 5.32” to 3.25”. The cars computer will function properly, the vehicle looks great and handles better, a win-win all around.

The copyright of the article Low Profile Tire Plus Sizing for Cars or Trucks in Auto Tech & Repair is owned by James Clausen. Permission to republish Low Profile Tire Plus Sizing for Cars or Trucks in print or online must be granted by the author in writing.

Plus Sizing Tires and Wheels - Low Profile Tires

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