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Plus sizing is the practice of replacing an automotive wheel with one of a larger diameter fitted with a new tire of lower aspect ratio so that the new tire has the same diameter and circumference as the original tire to prevent any changes in speedometer accuracy, torque and traction control, while reducing sidewall flex and--generally--increasing cornerning ability.
The number following the "plus" describes the number of inches which is added to the diameter of the rim. For example, plus one sizing means increasing the wheel by one inch--from a 15" to a 16" for example.
A 'plus zero' upgrade means changing to a wider tire size while using the same diameter wheel.
Changing to a wider tire requires reducing the aspect ratio (the second number in the sequence of numbers that describes the tire's size). Since the aspect ratio is a percentage which is used to calculate the height of the tire's sidewall, if follows that if a larger number is used for the width, a smaller number must be substituted if the final result is to remain the same—which is the objective of Plus sizing.
Plus sizing example
|Original tire||Plus zero||Plus one||Plus two|
These are simply examples and do not represent all of the possible combinations which could achieve the same result. For an R16 tire, 205/60, 225/55, 245/50 and 275/45 width/aspect ratio tires have essentially the same diameter.
- It is important to remember that actual dimensions of tires in the same size might differ slightly depending on tire brand, model, etc.
Larger tires improve handling and cornering, due to wider tread faces and stiffer sidewalls. Wider tires may decrease braking distances on dry pavement.
Larger wheels with lower profile tires are sometimes aesthetically or culturally desirable.
Larger wheels typically cost more. Wider tires tend to be more expensive because they are less common, and there is less competition between brands.
Performance improvements beyond what is achieved in a Plus One sizing are minimal.
Lower profile tires tend to have stiffer sidewalls, which might decrease riding comfort.
During winter, wider tires perform worse. Narrower tires exert more pressure on the contact patch, maintaining better road contact through snow.
Larger and wider wheels decreases fuel efficiency and increases consumption. A larger tire footprint can increase the time taken for "return to center" (steering) after taking a sharp turn.
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Some people claim larger wheels wear faster. Wheels with reduced sidewall heights may increase risk of damaged rims, breaking the bead, and/or damaged sidewalls. The increased width of the contact patch of wider tires may decrease the risk of hydroplaning.
Plus sizing tires may enhance the vehicle's value. Improving the vehicle from the factory specifications may increase value.
The additional height and weight of plus sized wheels may reduce vulnerability to rollovers, particularly by changing the center of gravity. During rapid tire deflation at speed, reduced sidewall height may decrease rollovers.
Total wheel weight (tire & rim) is part of the unsprung weight of the vehicle and will have a great effect on traction while traveling over uneven terrain as the wheel can respond quicker (lighter weight) to terrain changes. This allows the wheel to get back on the ground more quickly.
- Plus Size Calculator Compare the specs of your original tire with a plus size.
- Effects of Upsized Wheels and Tires Tested, Car and Driver, April, 2010
- Wheel Tech, Part III:Wheel Diameter's Effect on Performance, Tuner University.com
- Plus Size Wheels and Tires, Tirerack.com