285/40R21 Tire Size in Inches & Millimeters

Tire size “285/40R21” indicates your vehicle’s specifications based on tire width, aspect ratio, height, tire construction, diameter, radial and inches. These tire size numbers represent our tire sizes in both Metric (in millimeters) and Imperial (in inches) units of measurement to help find the correct tire size for your vehicle.

To convert tire size 285/40R21 to inches, you must know what these numbers represent.

So let us know 285/40R21 in simple words:

First number “285” indicates the tire’s width in millimeters.
Second number “40” refers to the aspect ratio, which is the height of the tire’s sidewall as a percentage of its width.
Third number “R21” refers to a tire that is designed to fit a 21-inch diameter rim.

Therefore, when converting the tire size 285/40R21 to inches, it can be concluded that the width of the tire will be approximately 11.2 inches ( 285 mm= 11.2 in), and the sidewall height will be approximately 4.5 inches (40% of 11.2 inches = 4.5 inches).

How to convert 285/40R21 into Inches (imperial)?

To convert tire size 285/40R21 to inches (Imperial) –

1. Width conversion: To convert it to inches, you can use a conversion factor of approximately 25.4 millimeters per inch.

  • 285 millimeters/25.4 = approximately 11.2 inches
    So, the width of the tire is approximately 11.2 inches.

2. Aspect Ratio Calculation: To calculate sidewall height in inches, you can use the tire width (from Step 1) and the aspect ratio:

  • Sidewall Height = Width (in inches) * Aspect Ratio / 100
  • Sidewall Height = 11.2 inches * 40/100 = approximately 4.5 inches
    So, the tire sidewall height is approximately 4.5 inches.

3. Rim Diameter: The “R” in the tire size stands for “radial construction” and the last number “21” indicates the rim diameter in inches. In this case, it’s already in inches.
So, the converted tire size “285/40R21” in inches (imperial) is approximately:

  • Width: 11.2 inches
  • Sidewall Height: 4.5 inches
  • Rim Diameter: 21 inches

How to convert 285/40R21 into Millimeters (metric)?

To convert the tire size 285/40R21 into millimeters (metric) –

1. Width Conversion: The first number in the tire size, “285“, represents the tire’s width in millimeters. Since it’s already in millimeters, there’s no need for conversion.

  • Width: 285millimeters

2. Aspect Ratio Calculation: The second number “40” is the aspect ratio, representing the tire’s sidewall height as a percentage of its width. To calculate the sidewall height in millimeters, you can use the tire’s width (from step 1) and the aspect ratio:

  • Sidewall height = Width (in millimeters) * Aspect Ratio / 100
  • Sidewall height = 285 millimeters * 40 / 100 = 114 mm millimeters
    So, the tire’s sidewall height is approximately 114 mm millimeters.

3. Rim Diameter: The “R” in the tire size stands for “radial construction” and the last number, “21” indicates the rim diameter in inches. To convert this to millimeters, you can use the conversion factor of approximately 25.4 millimeters per inch.

  • Rim diameter = 21 inches * 25.4 millimeters per inch = 533 mm millimeters
    So, the rim diameter is approximately 533 mm millimeters.

So, the converted tire size “285/40R21” in millimeters (metric) is approximately:

  • Width: 285 mm
  • Sidewall Height: 114 mm
  • Rim Diameter: 533 mm

Alternate Tire Size of 285/40R21 For replacement

Tire Size Overall Diameter Difference
295/35R21 29.1″ (739 mm) -2.9%
315/25R23 29.2″ (742 mm) -2.5%
305/30R22 29.3″ (743 mm) -2.4%
235/45R21 29.3″ (745 mm) -2.1%
265/35R22 29.3″ (745 mm) -2.1%
265/40R21 29.3″ (745 mm) -2.1%
305/35R21 29.4″ (747 mm) -1.8%
315/30R22 29.5″ (749 mm) -1.6%
275/35R22 29.6″ (751 mm) -1.3%
245/45R21 29.6″ (753 mm) -1.1%
275/40R21 29.6″ (753 mm) -1.1%
315/35R21 29.6″ (753 mm) -1.1%
285/35R22 29.9″ (759 mm) -0.3%
295/30R23 30″ (762 mm) 0.1%
255/40R22 30″ (763 mm) 0.3%
255/45R21 30″ (763 mm) 0.3%
295/35R22 30.1″ (765 mm) 0.5%
295/40R21 30.3″ (769 mm) 1.1%
265/45R21 30.4″ (771 mm) 1.3%
265/40R22 30.4″ (771 mm) 1.3%
305/35R22 30.4″ (773 mm) 1.6%
275/40R22 30.7″ (779 mm) 2.4%
315/35R22 30.7″ (779 mm) 2.4%
325/30R23 30.7″ (780 mm) 2.5%
275/45R21 30.7″ (781 mm) 2.6%

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