Plus sizing
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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.
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[edit] Plus sizing example
| Original tire | Plus zero | Plus one | Plus two |
|---|---|---|---|
| 205/60R16 | 225/55R16 | 215/55R17 | 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.
[edit] Advantages
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.
[edit] Disadvantages
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.
During the winter season, wider tires perform worse than a narrower tire. Think of it like a snowshoe. Snowshoes are wide, and allow the wearer to walk on top of the snow. Same applies to a tire. You want a narrower tire (within reason) that will cut *through* the snow and maintain contact with the road.
[edit] Controversial issues
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It has been claimed that larger wheels are likely to wear out quicker, are easier to damage due to less sidewall to give which can lead to damaged rims, breaking the bead, and/or damaged sidewalls, are more susceptible to hydroplaning due to the wider area of the tire creating less pressure per square inch causing it to act more like a water ski 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 Sport Utility Vehicles (SUVs) and trucks would be more vulnerable to rollovers with larger diameter rims (all else remaining the same), the center of gravity would change based on the total weight of the wheel (tire + rim). This could negatively affect center of gravity by raising or lower it. Also, during the rapid tire deflation, sidewall height is typically higher with OEM wheels and roll-over of SUVs & trucks usually occur during rapid tire pressure loss at speed. This was reason for the Ford / Firestone SUV roll-over debacle. It was later found out to be owner-driver issue where proper tire pressures were not maintained. This caused overheating of the tire and subsequent blow-outs.
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 of energy that has to be removed from the vehicle when braking which in theory, could cause the brakes to overheat. People claiming this fail to realize that the rotational energy of the tires and wheels is negligible during braking or acceleration compared to the mass of the vehicle they are supporting. Claims that these can cause insufficient braking are as false as claims that lighter flywheels will cause a car to accelerate more quickly.
Fact: 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 affect 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.
