Tire sizing
Encyclopedia
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.
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.
s 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.
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.
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 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.
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.
Tire
A tire or tyre is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground...
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
Speedometer
A speedometer is a gauge that measures and displays the instantaneous speed of a land vehicle. Now universally fitted to motor vehicles, they started to be available as options in the 1900s, and as standard equipment from about 1910 onwards. Speedometers for other vehicles have specific names...
accuracy, torque and traction control
Traction control
A traction control system , also known as anti-slip regulation , is typically a secondary function of the anti-lock braking system on production motor vehicles, designed to prevent loss of traction of driven road wheels...
.
The number following the "plus" describes the number of inches which is added to the diameter of the rim
Rim (wheel)
The rim of a wheel is the outer circular design of the metal on which the inside edge of the tire is mounted on vehicles such as automobiles. For example, on a bicycle wheel the rim is a large hoop attached to the outer ends of the spokes of the wheel that holds the tire and tube.In the 1st...
. 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
Aspect ratio
The aspect ratio of a shape is the ratio of its longer dimension to its shorter dimension. It may be applied to two characteristic dimensions of a three-dimensional shape, such as the ratio of the longest and shortest axis, or for symmetrical objects that are described by just two measurements,...
(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 | 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.
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 distanceBraking distance
Braking distance refers to the distance a vehicle will travel from the point where its brakes are fully applied to when it comes to a complete stop...
s 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
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.
Controversial issues
It has been claimed that larger tires are likely to wear out quicker, are easier to damage due to less sidewall to give which can lead to damaged rims, breaking a bead, and/or ripped 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 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 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.
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.