Aspect ratio is a ratio of width to height. In other words, it is a comparison of width to height or a relationship between the width and height.
A tire also has width and height therefore the aspect ratio of a tire gives a relationship between the two dimensions.
Sometimes, car owners have a need to install tire of different diameter from stock probably because they want to increase ride height or increase handling and performance.
There are many online tire size calculators some of which are not accurate.
It is good to know how other tire size parameters like section width, aspect ratio and rim size can be used to calculate section height and overall diameter of the wheel which includes the rim and the tire.
Using the tire size 255/45R20 in the image above, the dimensions are as follows:
Width = 255 mm
Aspect ratio = 45
Rim size = 20 inches (converting to millimeters, we have 20 × 25.4 = 508 mm).
What is tire width?
The width of a tire is the size from the outer edge of a sidewall to the edge of the other sidewall. It is usually in millimetres.
What is aspect ratio?
This is the ratio of the section height to the section width, the height being at the numerator. It is the value after the slash in the tire dimension marking and the value is a percentage. For instance, a 255/45R20 has a 45 aspect ratio. This means that the section height is 45% of the section width.
What is wheel diameter?
This indicates the distance from one end of the wheel to the other. The wheel includes the rim and the tire. The bigger the wheel diameter, the higher the ground clearance of the vehicle.
How to obtain tire section height
From the tire size example 255/45R20, we can obtain the section height from the information that we already have.
As we have earlier said, the aspect ratio 45 means that the section height is 45% of the section width.
Section height = (45/100) × 255 = 114.75 mm
How to obtain wheel diameter
Let's recall that the overall diameter of the wheel includes the rim which is at the center of two sides of the tire sections. This means that the section height that we obtained earlier will be multiplied by 2.
Again a 20 inches rim = 20 × 25.4 = 508 mm.
Therefore, wheel diameter = (114.75 × 2) + (20 × 25.4) = 229.5 + 50.8 = 280.3 mm.
280.3mm is the overall diameter of the wheel.If we are to convert to inches = 280.3/25.4 = 11.04 inches.
These calculation steps are same you should follow if you want to compare the tire diameter of different tires. A larger diameter or tire will give you higher ride height and vice versa.
Note that a larger aspect ratio value does not always translate to a larger diameter tire since it depends on the value of the section width.