315/40R21 in Inches

A 315/40R21 tire has a 30.9-inch diameter, a 12.4-inch width, and is designed for a 21-inch rim.

In high flotation terms, this tire is labeled as 30.9x12.4R21.

12.4"

30.9"

97.1"

21"

652.2 revs/mile

315/40R21 Tire Overview

Tirelib.com

315/40R21 is a tire size with the following dimensions:

315 is the width of the tire in millimeters. This number also indicates the appropriate width of the rim on which the tire should be mounted.
40 is the aspect ratio, which is the ratio of the tire's height to its width. This affects the tire's handling and ride comfort
R indicates the tire's construction type, meaning it has a radial construction. This provides stability and precision to the tire.
21 is the diameter of the wheel in inches, on which the tire is to be mounted.

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The height (or diameter) of a 315/40R21 tire is 30.9 inches

The width of a 315/40R21 tire is 315 mm, which is about 12.4 inches.

A 315/40R21 tire fits on a rim that is 21 inches in diameter.

The 40 in a 315/40R21 tire refers to the aspect ratio, meaning the height of the tire's sidewall is 40% of its width.

Typically, it's recommended to choose a tire with a diameter (height) within 3% of the original equipment manufacturer (OEM) tire size.

However, keep in mind that this is a general guideline and not an absolute rule.

Here are some tire sizes that are compatible with the 315/40R21: