315/30R21 Tire Size in Inches and Millimeters

315/30R21 tires have a diameter of 28.5" (723 mm), a tread width of 12.4" (315 mm), and a wheel diameter of 21" (533.4 mm).The sidewall of the tire is 3.7" (95 mm), and the tire's circumference is 89.5" (2270.2 mm). This means a 315/30R21 tire will make make 708 revolutions per mile, or 440 revolutions per kilometer.

What Does 315/30R21 Mean?

Frequently Asked Questions About 315/30R21 Tires

How tall is a 315/30R21 tire?

A 315/30R21 tire has an approximate height, also known as the overall diameter, of 28.5 inches (723 mm). This height can vary slightly depending on the brand and model of the tire, as well as the amount of air pressure in the tire.

How wide is a 315/30R21 tire?

A 315/30R21 tire has a width of approximately 12.4 inches (315 millimeters). The width of the tire is the first number in the tire size designation, which in this case is “315”. It’s important to note that the actual width of the tire may vary slightly depending on the brand and model of the tire, and the width may also be affected by the amount of air pressure in the tire.

Understanding Tire Speed Ratings

When choosing the right tires for your vehicle, understanding the speed rating is crucial. But what exactly is a speed rating, and why does it matter?

What is a Tire Speed Rating?

A tire's speed rating indicates the maximum speed a tire can safely sustain over time. It's a manufacturer's assurance of tire performance, marked by a letter on the tire's sidewall. This rating is determined under controlled laboratory conditions and is part of a broader standard established by industry entities like the European Tyre and Rim Technical Organisation (ETRTO) and the U.S. Department of Transportation.

Common Speed Ratings Explained

1. P-Rated: Up to 94 mph - Suitable for smaller sedans or light-duty vehicles.
2. Q-Rated: Up to 100 mph - Often found on SUVs and light trucks with consistent highway use.
3. R-Rated: Up to 106 mph - Typically for medium-duty trucks and vans.
4. S-Rated: Up to 112 mph - Common for family sedans and larger vehicles with moderate performance.
5. T-Rated: Up to 118 mph - Optimal for family vehicles, including minivans and some coupes.

Why is the Speed Rating Important?

The speed rating is not just about how fast you can legally drive; it's about safety and performance:

• Ensures Safety: It guarantees that the tire can handle certain speeds without overheating and failing, which can prevent blowouts.
• Optimizes Performance: Higher ratings often indicate better handling characteristics, important for maintaining vehicle stability.
• Legal and Insurance Considerations: Matching the speed rating suggested by your vehicle manufacturer can be essential for insurance and warranty purposes.

When choosing tires, always refer to your vehicle’s manual to ensure compatibility. Remember, fitting tires with a lower speed rating than recommended can compromise safety and performance, while higher ratings might not provide extra benefits unless you frequently drive at high speeds. By understanding these nuances, you can make an informed choice that suits your driving habits while keeping safety a top priority.

Alternate 315/30R21 Replacement Tire Sizes

Upsizing your wheels or tires? Replacement tires must be within 3% of the diametor of the original tire. The following sizes fall within that range - click on any size to see detailed info about that tire and wheel combination.