225/40R18 Tire Size in Inches & Millimeters

Tire size “225/40R18” 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 225/40R18 to inches, you must know what these numbers represent.

So let us know 225/40R18 in simple words:

First number “225” 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 “R18” refers to a tire that is designed to fit a 18-inch diameter rim.

Therefore, when converting the tire size 225/40R18 to inches, it can be concluded that the width of the tire will be approximately 8.9 inches ( 225 mm= 8.9 in), and the sidewall height will be approximately 3.5 inches (40% of 8.9 inches = 3.5 inches).

How to convert 225/40R18 into Inches (imperial)?

To convert tire size 225/40R18 to inches (Imperial) –

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

  • 225 millimeters/25.4 = approximately 8.9 inches
    So, the width of the tire is approximately 8.9 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 = 8.9 inches * 40/100 = approximately 3.5 inches
    So, the tire sidewall height is approximately 3.5 inches.

3. Rim Diameter: The “R” in the tire size stands for “radial construction” and the last number “18” indicates the rim diameter in inches. In this case, it’s already in inches.
So, the converted tire size “225/40R18” in inches (imperial) is approximately:

  • Width: 8.9 inches
  • Sidewall Height: 3.5 inches
  • Rim Diameter: 18 inches

How to convert 225/40R18 into Millimeters (metric)?

To convert the tire size 225/40R18 into millimeters (metric) –

1. Width Conversion: The first number in the tire size, “225“, represents the tire’s width in millimeters. Since it’s already in millimeters, there’s no need for conversion.

  • Width: 225millimeters

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 = 225 millimeters * 40 / 100 = 90 mm millimeters
    So, the tire’s sidewall height is approximately 90 mm millimeters.

3. Rim Diameter: The “R” in the tire size stands for “radial construction” and the last number, “18” 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 = 18 inches * 25.4 millimeters per inch = 457 mm millimeters
    So, the rim diameter is approximately 457 mm millimeters.

So, the converted tire size “225/40R18” in millimeters (metric) is approximately:

  • Width: 225 mm
  • Sidewall Height: 90 mm
  • Rim Diameter: 457 mm

Alternate Tire Size of 225/40R18 For replacement

Tire Size Overall Diameter Difference
135/60R18 24.4″ (619 mm) -2.8%
225/30R19 24.4″ (619 mm) -2.8%
235/35R18 24.4″ (621 mm) -2.5%
205/40R18 24.4″ (621 mm) -2.5%
235/30R19 24.6″ (625 mm) -1.9%
245/35R18 24.8″ (629 mm) -1.3%
285/30R18 24.8″ (629 mm) -1.3%
215/40R18 24.8″ (629 mm) -1.3%
295/25R19 24.8″ (631 mm) -0.9%
245/30R19 24.8″ (631 mm) -0.9%
215/35R19 24.9″ (633 mm) -0.6%
125/70R18 24.9″ (633 mm) -0.6%
305/25R19 25″ (635 mm) -0.3%
255/35R18 25″ (635 mm) -0.3%
295/30R18 25″ (635 mm) -0.3%
255/30R19 25.1″ (637 mm) 0%
215/30R20 25.1″ (638 mm) 0.2%
225/35R19 25.2″ (641 mm) 0.6%
305/30R18 25.2″ (641 mm) 0.6%
205/45R18 25.2″ (641 mm) 0.6%
315/25R19 25.2″ (641 mm) 0.6%
265/35R18 25.3″ (643 mm) 0.9%
265/30R19 25.3″ (643 mm) 0.9%
155/60R18 25.3″ (643 mm) 0.9%
225/30R20 25.4″ (644 mm) 1.1%
115/70R19 25.4″ (645 mm) 1.3%
135/60R19 25.4″ (645 mm) 1.3%
145/65R18 25.4″ (645 mm) 1.3%
235/40R18 25.4″ (645 mm) 1.3%
275/25R20 25.4″ (646 mm) 1.4%
235/35R19 25.5″ (647 mm) 1.6%
315/30R18 25.5″ (647 mm) 1.6%
135/70R18 25.5″ (647 mm) 1.6%
275/30R19 25.6″ (649 mm) 1.9%
275/35R18 25.6″ (649 mm) 1.9%
235/30R20 25.6″ (650 mm) 2%
285/25R20 25.6″ (650 mm) 2%
335/25R19 25.6″ (651 mm) 2.2%
215/45R18 25.6″ (651 mm) 2.2%
245/40R18 25.7″ (653 mm) 2.5%
215/40R19 25.8″ (655 mm) 2.8%
285/30R19 25.8″ (655 mm) 2.8%
245/35R19 25.8″ (655 mm) 2.8%
295/25R20 25.8″ (656 mm) 3%
245/30R20 25.8″ (656 mm) 3%

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