What is the aspect ratio of a tire? It is an important characteristic often overlooked when selecting new tires for your vehicle. However, understanding this aspect of tire construction can greatly affect the performance and handling of your car. Let’s learn more about the matter with our article.
In this article:
Aspect Ratio Tire Size Meaning
The tire aspect ratio refers to the height of a tire sidewall to its width, expressed as a percentage. It is a critical parameter affecting many tire performance aspects, including ride comfort, handling, and fuel efficiency.
It is also an essential factor in determining the overall diameter of the tire, which directly impacts speedometer accuracy.
Where To Find Aspect Ratio
You can find the aspect ratio in the tire size markings on the sidewall of the tire. The tire size markings consist of a number series and letters that provide information about the tire’s size, construction, and performance characteristics.
The aspect ratio is represented by a two-digit number following the tire width measurement, separated by a slash (/), in the tire size markings on the sidewall of the tire.
For example, in a tire size marking of P225/60R16, the aspect ratio is 60.
Note that this value is measured when the tire is properly inflated to the recommended PSI, mounted on a required rim, and carrying no load.
How To Calculate Tire Aspect Ratio
Calculating this ratio is a simple process that requires basic math skills. You can figure it by dividing the sidewall height by tire width and multiplying the result by 100. The height is calculated by multiplying the tire’s width by the aspect ratio.
Take my BMW 330 Gran Turismo for example. It has a tire size of P225/60R16. The aspect ratio of my tire is 60, which means that the tire height is 60% of the tire’s width. And the tire width is 225.
Of course, it takes me less than a second to notice the “60” number in the string on my tire. But knowing this formula, I can calculate other specs that are not printed on the tire.
In this case, for instance, the sidewall height would be calculated as follows:
- Tire Sidewall Height = 225 x 0.60 = 135 mm (~5.3 inches)
How Aspect Ratio Affects Tire Performance
As mentioned before, this ratio has a great impact on the overall tire performance, including speed, fuel economy, handling ability, and hydroplaning resistance.
Tires with a lower aspect ratio tend to provide better high-speed handling and stability than those with a higher aspect ratio. This is because these models have shorter sidewalls, which provide less flex and deformation at maximum speeds.
As a result, they provide better cornering stability, faster acceleration, and shorter stopping distances.
Tires with a lower aspect ratio tend to have lower rolling resistance, which can improve fuel economy. Rolling resistance refers to the energy required to keep the tire rolling, and tires with a lower aspect ratio have less sidewall flex, which reduces rolling resistance. Still, this can also result in harsher ride quality.
Reduced flexible sidewall height enhances tire structural integrity and boosts radial, lateral, and circumferential stiffness in tires with lower aspect ratios. The performance of corners is improved by increased lateral stiffness.
In particular, when combined with cornering, increased circumferential rigidity enhances handling during deceleration and acceleration. Low-ratio tires with shorter sidewalls allow for an increase in rim diameter without an increase in outer wheel diameter. Meanwhile, a larger rim size is preferred for ABS (anti-lock braking systems).
When water collects as a film beneath the footprint, lifting a tire off the road and reducing traction, it is known as hydroplaning. The area of the tire footprint grows when a tire’s aspect ratio decreases or its breadth widens.
The wider footprint area decreases the contact patch’s average pressure. Lower aspect ratio tires provide less hydroplaning resistance than high aspect ratio tires because of the close relationship between footprint pressure and hydroplaning resistance. To lessen dry cornering difficulty while increasing hydroplaning resistance, the tread void can be raised.
I have tested with two tire sets, one with 70 aspect ratio and one with an ultra-low ratio, both from Bridgestone. After 6 months of use, the biggest difference is a great decrease in rolling resistance (say, 10%) and noise level. Also, the lower-profile tires come with a softer tread compound, so I notice more even treadwear than on the 70 series.
What Is A Good Aspect Ratio For Tires? What If It’s Too Low Or Too High?
Most automobiles employ a 75-aspect ratio, which is the industry standard. That being said, a good aspect ratio for tires depends on the vehicle type and driving conditions.
A lower aspect ratio (40 or 45) is ideal for sports vehicles or high-performance cars requiring precise handling and responsive steering. These tires have shorter sidewalls, giving better grip and stability during high-speed cornering. Meanwhile, a higher rate (65 or 70) is geared towards passenger vehicles or SUVs, providing a comfortable ride with better shock absorption.
When the aspect ratio is too low, it can result in a harsh ride, poor handling, and a higher risk of wheel and tire damage from road irregularities. The lower sidewall height means there is less cushioning, and it’s more susceptible to damage from potholes, bumps, and debris on the road. Moreover, these tire types may also wear out faster, leading to more frequent replacements and increased costs.
On the flip side, if the aspect ratio is too high, it can result in poor handling and reduced stability. Tires with higher aspect ratios have longer sidewalls, which can flex more during cornering and result in imprecise steering and handling. Taller sidewalls can also cause a higher rollover risk in SUVs, light trucks, and other top-heavy vehicles.
Other Measurement For Tire Ratio
Width and diameter are two of the most critical measurements beyond the tire aspect ratio that you need to pay attention to.
Tire width refers to the distance between the two outer edges of the tire’s sidewall, measured in millimeters. It is the first number listed in the tire size marking on the sidewall before the slash (/) that separates it from the aspect ratio. For example, a tire size of 215/60R16 has a width of 215mm.
This measurement affects the tire’s contact patch, which is the area that makes contact with the road. A wider tire provides a wider tread contact patch, which can improve traction and handling. However, larger-diameter tires can also be more expensive and may have higher rolling resistance, affecting fuel efficiency.
Tire diameter represents the overall height of the tire, from the top tread to the bottom of the tire bead, measured in inches. The sidewall bears a tire size marking, with this one being the final number. A 215/60R16 tire, for instance, has a 16-inch wheel diameter.
This number influences the circumference of the tire as a whole, which directly affects the accuracy of the speedometer and gear ratios. As a result of the greater gear ratio that a wider tire diameter offers, driving down the RPMs on the highway and improving fuel economy are both possible outcomes.
Factor To Consider When Choosing Type Tire
Besides the aspect tire ratio, there are several factors to consider when selecting the perfect tires for your vehicle.
- Type of vehicle and intended use
- Climate and weather conditions
- Tread pattern and wear
- Load capacities and speed ratings
- Fuel efficiency
- Handling and drivetrain performance
- Noise level
- Brand and warranty
- Availability and installation options
Frequently Asked Questions
Is It Ok To Change Tire Aspect Ratio?
Yes, it is completely fine to alter the aspect ratio of your tire. It’s crucial to keep in mind, though, that modifying it will also change the tire’s diameter. The aspect ratio will cause the diameter to decrease. This may be useful if you want to lift your car since it is too close to the ground.
Can I Use 60 Tires Instead Of 65?
Yes. It is generally safe to use 60 grades instead of 65, but there are some factors to consider. The tire aspect ratio 60 vs 65 affects the overall diameter of the tire, so a 60 tire will be slightly smaller in diameter than a 65 tire. This can result in a slightly lower ride height, which can affect handling and clearance.
Can I Use 45 Tires Instead Of 55?
No. It is not recommended to use 45 aspect ratio tires instead of 55, as the difference in tire aspect ratio 45 vs 55 can significantly affect the overall diameter and actual tire performance.
The Bottom Line
Choosing the right aspect ratio is essential to ensure the proper tire fit, handling, and performance of your vehicle. With the correct tires, you can enjoy a safer and more comfortable driving experience in any road condition. Suppose you are still unsure; consult with a tire professional or follow tire manufacturers to pick the right one.