P315/40R18 Tire Size in Inches & Millimeters

Tire size “315/40R18” 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 P315/40R18 to inches, you must know what these numbers represent.

So let us know P315/40R18 in simple words:

First number “315” 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 P315/40R18 to inches, it can be concluded that the width of the tire will be approximately 12.4 inches ( 315 mm= 12.4 in), and the sidewall height will be approximately 5 inches (40% of 12.4 inches = 5 inches).

How to convert P315/40R18 into Inches (imperial)?

To convert tire size P315/40R18 to inches (Imperial) –

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

  • 315 millimeters/25.4 = approximately 12.4 inches
    So, the width of the tire is approximately 12.4 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 = 12.4 inches * 40/100 = approximately 5 inches
    So, the tire sidewall height is approximately 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 “P315/40R18” in inches (imperial) is approximately:

  • Width: 12.4 inches
  • Sidewall Height: 5 inches
  • Rim Diameter: 18 inches

How to convert P315/40R18 into Millimeters (metric)?

To convert the tire size P315/40R18 into millimeters (metric) –

1. Width Conversion: The first number in the tire size, “315“, represents the tire’s width in millimeters. Since it’s already in millimeters, there’s no need for conversion.

  • Width: 315millimeters

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 = 315 millimeters * 40 / 100 = 126 mm millimeters
    So, the tire’s sidewall height is approximately 126 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 “P315/40R18” in millimeters (metric) is approximately:

  • Width: 315 mm
  • Sidewall Height: 126 mm
  • Rim Diameter: 457 mm

Alternate Tire Size of P315/40R18 For replacement

Tire Size Overall Diameter Difference
225/40R20 27.1″ (688 mm) -3%
145/80R18 27.1″ (689 mm) -2.8%
165/70R18 27.1″ (689 mm) -2.8%
295/35R19 27.1″ (689 mm) -2.8%
345/30R19 27.2″ (691 mm) -2.5%
305/30R20 27.2″ (692 mm) -2.4%
215/55R18 27.3″ (693 mm) -2.3%
235/50R18 27.3″ (693 mm) -2.3%
295/40R18 27.3″ (693 mm) -2.3%
175/60R19 27.3″ (693 mm) -2.3%
155/60R20 27.3″ (694 mm) -2.1%
265/35R20 27.3″ (694 mm) -2.1%
235/45R19 27.4″ (695 mm) -2%
265/40R19 27.4″ (695 mm) -2%
265/45R18 27.4″ (695 mm) -2%
145/65R20 27.4″ (696 mm) -1.8%
235/40R20 27.4″ (696 mm) -1.8%
355/30R19 27.4″ (697 mm) -1.7%
305/35R19 27.4″ (697 mm) -1.7%
315/30R20 27.5″ (698 mm) -1.6%
345/35R18 27.5″ (699 mm) -1.4%
175/55R20 27.6″ (700 mm) -1.3%
275/35R20 27.6″ (700 mm) -1.3%
305/40R18 27.6″ (701 mm) -1.1%
155/70R19 27.6″ (701 mm) -1.1%
215/45R20 27.6″ (702 mm) -1%
275/40R19 27.7″ (703 mm) -0.8%
145/85R18 27.7″ (703 mm) -0.8%
205/60R18 27.7″ (703 mm) -0.8%
245/50R18 27.7″ (703 mm) -0.8%
245/45R19 27.7″ (703 mm) -0.8%
315/35R19 27.7″ (703 mm) -0.8%
325/30R20 27.7″ (704 mm) -0.7%
195/50R20 27.7″ (704 mm) -0.7%
245/40R20 27.7″ (704 mm) -0.7%
225/55R18 27.8″ (705 mm) -0.6%
155/80R18 27.8″ (705 mm) -0.6%
275/45R18 27.8″ (705 mm) -0.6%
165/60R20 27.8″ (706 mm) -0.4%
285/35R20 27.9″ (708 mm) -0.1%
225/45R20 28″ (710 mm) 0.1%
155/65R20 28″ (710 mm) 0.1%
335/30R20 28″ (710 mm) 0.1%
285/40R19 28″ (711 mm) 0.3%
255/40R20 28″ (712 mm) 0.4%
255/45R19 28.1″ (713 mm) 0.6%
285/45R18 28.1″ (713 mm) 0.6%
255/50R18 28.1″ (713 mm) 0.6%
295/35R20 28.1″ (714 mm) 0.7%
145/80R19 28.1″ (715 mm) 0.8%
235/55R18 28.1″ (715 mm) 0.8%
345/30R20 28.2″ (716 mm) 1%
235/50R19 28.3″ (719 mm) 1.4%
295/40R19 28.3″ (719 mm) 1.4%
235/45R20 28.3″ (720 mm) 1.6%
265/40R20 28.3″ (720 mm) 1.6%
155/85R18 28.4″ (721 mm) 1.7%
265/45R19 28.4″ (721 mm) 1.7%
305/35R20 28.4″ (722 mm) 1.8%
295/45R18 28.5″ (723 mm) 2%
265/50R18 28.5″ (723 mm) 2%
345/35R19 28.5″ (725 mm) 2.3%
225/60R18 28.6″ (727 mm) 2.5%
245/55R18 28.6″ (727 mm) 2.5%
315/35R20 28.7″ (728 mm) 2.7%
275/40R20 28.7″ (728 mm) 2.7%
245/45R20 28.7″ (728 mm) 2.7%
245/50R19 28.7″ (729 mm) 2.8%

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