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, power 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 is a change to a wider tire but using the same diameter wheel.

Changing to a wider tire requires reducing the aspect ratio (which is 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

These are simply examples and do not represent all of the possible combinations which could achieve the same result. For an R16 tire, 190/65, 205/60, 225/55, 245/50 and 275/45 width/aspect ratio tires have essentially the same diameter.

Advantages to Plus Sizing

Advantages include greater handling and cornering abilities of the car, which is a result of the wider tread face and stiffer sidewall of the larger tire. Wider tires may decrease braking distances on dry pavement.

Larger wheels change the appearance of the vehicle and together with a lower profile tire produce an effect which some people find attractive.

Disadvantages to Plus Sizing

Larger wheels will cost more and wider tires tend to be more expensive because they are less common and have less competition between different brands to drive down the price.

Performance improvements beyond what is achieved in a Plus One sizing are likely to be minimal.

The use of lower profile tires which tend to have a stiffer sidewall might produce a decrease in riding comfort.

Controversial issues

It has been claimed that larger tires are likely to wear out quicker, are easier to damage, are more susceptible to hydroplaning and, therefore, are less effective in wet weather, however, there has been little empirical evidence to support this belief.

Some car owners believe that the plus sizing may enhance the vehicle's value, others argue the altering the vehicle from the factory specifications may be a negative.

Some believe that SUVs and Trucks would be more vulnerable to rollovers, whereas, the fact that the center of gravity of the vehicle essentially remains unchanged would seem to negate this claim. Ride comfort will also have to be sacrificed in order to deal with performance enhancement.

It is also claimed that plus sizing may negatively impact acceleration performance and fuel economy. The argument is that even though the overall wheel diameter measured at the tread stays the same, moving the heavy rim component of the wheel outwards from the axis of rotation of the wheel results in an increase in rotational mass, assuming the choice of materials is not altered. This amounts to an increase of 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 a higher amount energy that has to be removed from the vehicle when braking which in theory, could cause the brakes to overheat.

Countering this is the claim that by replacing a heavier steel wheel with a lighter aluminum alloy wheel, the aforementioned performance degradation due to increased rotational inertia can be offset and may, in fact, produce an overall decrease in tire-wheel mass.

Whether or not either of the preceding claims results in any significant difference is another issue to be considered.

ee also

*Tire code

External links

* [http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=25 The Plus Size Concept]
* [http://www.roadandtravel.com/consumerreports/plussizetires.htm Tire Sizing Advantages/Disadvantages]
* [http://www.tiresizecalculator.info Tire / Wheel Size Calculator]
* [http://www.miata.net/garage/tirecalc.html Java based Tire / Wheel Size Calculator]