Plus sizing

Plus sizing is the practice of changing a specific tire to a larger size while compensating with reductions in other aspects of the tire's size 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.

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.

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
205/60R16	225/55R16	225/50R17	245/40R18

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.

Advantages

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.

Disadvantages

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.

Controversial issues

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 increase the risk of hydroplaning.

Plus sizing tires may enhance the vehicle's value, but altering the vehicle from the factory specifications may reduce value.

The additional height and weight of plus sized wheels may increase vulnerability to rollovers, particularly by changing the center of gravity. During rapid tire deflation at speed, reduced sidewall height may encourage rollovers.

Whilst some research suggests that Plus sizing may reduce acceleration performance and fuel economy, others would argue that the fewer revolutions required to cover the same distance makes poorer fuel economy somewhat questionable. While overall wheel diameter measured at the tread is unchanged, rotating heavier rims outwards from the axis of rotation increases rotating mass. This increases the energy transfer needed to accelerate or decelerate the wheel. When a vehicle's acceleration is limited by engine power, the corresponding result is a decrease in acceleration performance. Higher rotational mass also means increased inertia, potentially increasing heat.

Acceleration is a function of total vehicle mass & force at the tire patch. While wheel weight and mass moment of inertia does have a slight effect on vehicular acceleration/deceleration via inertia, the larger effect of increasing wheel diameter is torque loss. If you install a wheel/tire combo that's 20% larger in diameter, you will have 20% higher output/input effective gear ratio (like shifting into a higher gear), for accelerating and braking. This could make the vehicle accelerate more slowly depending on the torque available, but will also make the brakes feel weaker (more difficult to lock up). 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.

See also

• Tire code

External links

• Tire Calculator Using Metric or Inches