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.



Spec | Inches | Metric |
---|---|---|
Diameter | 28.5" | 723 mm |
Width | 12.4" | 315 mm |
Wheel | 21" | 533.4 mm |
Sidewall | 3.7" | 95 mm |
Circum | 89.5" | 2270.2 mm |
Revolutions | 708 per mile | 440 per km |
What Does 315/30R21 Mean?
The tire size 315/30R21 is a common size used on many trucks, SUVs, and other larger vehicles.
Here is what each part of the size designation represents:
- 315: This number represents the width of the tire in millimeters. In this case, the tire is 315 millimeters wide.
- 30: This number represents the aspect ratio of the tire, which is the height of the tire's sidewall as a percentage of its width. In this case, the sidewall height is 30% of the tire's width.
- R: This letter indicates the tire's construction type. “R” stands for radial construction, which is the most common type of tire construction today.
- 21: This number represents the diameter of the wheel that the tire is designed to fit on, in inches. In this case, the tire is designed to fit on a 21-inch wheel.

How To Convert 315/30R21 Into Inches
The 315/30R21 conversion is fairly simple. The first number in the tire size, 315, represents the width of the tire in millimeters. To convert this to inches, we divide 315 by 25.4 (since there are 25.4 millimeters in an inch). This gives us approximately 12.4 inches.
The second number in the tire size, 30, represents the aspect ratio or the height of the tire as a percentage of its width. To calculate the height in inches, we multiply 12.4 by 0.30, which gives us approximately 3.7 inches.
Finally, the last number in the tire size, 21, represents the diameter of the wheel rim in inches. Therefore, the tire size 315/30R21 can be expressed in inches as roughly 12.4 inches wide, 3.7 inches tall, and designed to fit a 21-inch wheel rim.
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
- P-Rated: Up to 94 mph - Suitable for smaller sedans or light-duty vehicles.
- Q-Rated: Up to 100 mph - Often found on SUVs and light trucks with consistent highway use.
- R-Rated: Up to 106 mph - Typically for medium-duty trucks and vans.
- S-Rated: Up to 112 mph - Common for family sedans and larger vehicles with moderate performance.
- 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.
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.
Size | Diameter | Width | Wheel |
---|---|---|---|
145/80R19 | 28.1" | 715 mm | 19 mm |
145/80R20 | 29.1" | 740 mm | 20 mm |
145/85R18 | 27.7" | 703 mm | 18 mm |
155/65R20 | 28" | 710 mm | 20 mm |
155/80R18 | 27.8" | 705 mm | 18 mm |
155/80R19 | 28.8" | 731 mm | 19 mm |
155/85R18 | 28.4" | 721 mm | 18 mm |
155/90R17 | 28" | 712 mm | 17 mm |
155/90R18 | 29" | 737 mm | 18 mm |
165/60R20 | 27.8" | 706 mm | 20 mm |
165/90R17 | 28.7" | 730 mm | 17 mm |
175/80R18 | 29" | 737 mm | 18 mm |
195/50R20 | 27.7" | 704 mm | 20 mm |
195/80R16 | 28.3" | 718 mm | 16 mm |
205/60R18 | 27.7" | 703 mm | 18 mm |
205/75R16 | 28.1" | 714 mm | 16 mm |
205/80R16 | 28.9" | 734 mm | 16 mm |
215/45R20 | 27.6" | 702 mm | 20 mm |
215/65R17 | 28" | 712 mm | 17 mm |
215/70R16 | 27.9" | 708 mm | 16 mm |
215/70R17 | 28.9" | 734 mm | 17 mm |
215/75R15 | 27.7" | 703 mm | 15 mm |
215/75R16 | 28.7" | 728 mm | 16 mm |
215/80R15 | 28.5" | 725 mm | 15 mm |
225/45R20 | 28" | 710 mm | 20 mm |
225/55R18 | 27.8" | 705 mm | 18 mm |
225/55R19 | 28.8" | 731 mm | 19 mm |
225/60R17 | 27.6" | 702 mm | 17 mm |
225/60R18 | 28.6" | 727 mm | 18 mm |
225/65R17 | 28.5" | 724 mm | 17 mm |
225/70R16 | 28.4" | 722 mm | 16 mm |
225/75R15 | 28.3" | 719 mm | 15 mm |
225/75R16 | 29.3" | 744 mm | 16 mm |
225/80R15 | 29.2" | 741 mm | 15 mm |
235/45R20 | 28.3" | 720 mm | 20 mm |
235/50R19 | 28.3" | 719 mm | 19 mm |
235/50R20 | 29.3" | 744 mm | 20 mm |
235/55R18 | 28.1" | 715 mm | 18 mm |
235/55R19 | 29.2" | 741 mm | 19 mm |
235/60R17 | 28.1" | 714 mm | 17 mm |
235/60R18 | 29.1" | 739 mm | 18 mm |
235/65R16 | 28" | 712 mm | 16 mm |
235/65R17 | 29.1" | 738 mm | 17 mm |
235/70R15 | 28" | 711 mm | 15 mm |
235/70R16 | 29" | 736 mm | 16 mm |
235/75R15 | 28.9" | 733 mm | 15 mm |
245/25R24 | 28.8" | 732 mm | 24 mm |
245/30R22 | 27.8" | 707 mm | 22 mm |
245/35R21 | 27.8" | 705 mm | 21 mm |
245/35R22 | 28.8" | 731 mm | 22 mm |
245/40R20 | 27.7" | 704 mm | 20 mm |
245/40R21 | 28.7" | 729 mm | 21 mm |
245/45R19 | 27.7" | 703 mm | 19 mm |
245/45R20 | 28.7" | 728 mm | 20 mm |
245/50R18 | 27.7" | 703 mm | 18 mm |
245/50R19 | 28.7" | 729 mm | 19 mm |
245/55R17 | 27.6" | 702 mm | 17 mm |
245/55R18 | 28.6" | 727 mm | 18 mm |
245/60R17 | 28.6" | 726 mm | 17 mm |
245/70R15 | 28.5" | 725 mm | 15 mm |
255/25R24 | 29.1" | 738 mm | 24 mm |
255/30R22 | 28.1" | 713 mm | 22 mm |
255/35R21 | 28" | 711 mm | 21 mm |
255/35R22 | 29" | 737 mm | 22 mm |
255/40R20 | 28" | 712 mm | 20 mm |
255/40R21 | 29" | 737 mm | 21 mm |
255/45R19 | 28.1" | 713 mm | 19 mm |
255/45R20 | 29.1" | 738 mm | 20 mm |
255/50R18 | 28.1" | 713 mm | 18 mm |
255/50R19 | 29.1" | 739 mm | 19 mm |
255/55R17 | 28" | 712 mm | 17 mm |
255/55R18 | 29" | 737 mm | 18 mm |
255/60R16 | 28" | 712 mm | 16 mm |
255/60R17 | 29.1" | 738 mm | 17 mm |
255/65R15 | 28.1" | 713 mm | 15 mm |
255/65R16 | 29.1" | 738 mm | 16 mm |
255/70R15 | 29.1" | 739 mm | 15 mm |
265/25R23 | 28.2" | 716 mm | 23 mm |
265/25R24 | 29.2" | 742 mm | 24 mm |
265/30R22 | 28.3" | 719 mm | 22 mm |
265/35R21 | 28.3" | 719 mm | 21 mm |
265/40R20 | 28.3" | 720 mm | 20 mm |
265/45R19 | 28.4" | 721 mm | 19 mm |
265/50R18 | 28.5" | 723 mm | 18 mm |
265/55R17 | 28.5" | 724 mm | 17 mm |
265/60R16 | 28.5" | 724 mm | 16 mm |
275/23R24 | 29" | 736 mm | 24 mm |
275/30R22 | 28.5" | 725 mm | 22 mm |
275/35R21 | 28.5" | 725 mm | 21 mm |
275/40R19 | 27.7" | 703 mm | 19 mm |
275/40R20 | 28.7" | 728 mm | 20 mm |
275/45R18 | 27.8" | 705 mm | 18 mm |
275/45R19 | 28.8" | 731 mm | 19 mm |
275/50R17 | 27.9" | 708 mm | 17 mm |
275/50R18 | 28.9" | 733 mm | 18 mm |
275/55R16 | 27.9" | 708 mm | 16 mm |
275/55R17 | 28.9" | 734 mm | 17 mm |
275/60R15 | 28" | 711 mm | 15 mm |
275/60R16 | 29" | 736 mm | 16 mm |
285/30R21 | 27.8" | 705 mm | 21 mm |
285/30R22 | 28.8" | 731 mm | 22 mm |
285/35R20 | 27.9" | 708 mm | 20 mm |
285/35R21 | 28.9" | 733 mm | 21 mm |
285/40R19 | 28" | 711 mm | 19 mm |
285/40R20 | 29" | 736 mm | 20 mm |
285/45R18 | 28.1" | 713 mm | 18 mm |
285/45R19 | 29.1" | 739 mm | 19 mm |
285/50R18 | 29.3" | 743 mm | 18 mm |
295/25R22 | 27.8" | 707 mm | 22 mm |
295/30R21 | 28" | 711 mm | 21 mm |
295/30R22 | 29" | 737 mm | 22 mm |
295/35R20 | 28.1" | 714 mm | 20 mm |
295/35R21 | 29.1" | 739 mm | 21 mm |
295/40R19 | 28.3" | 719 mm | 19 mm |
295/40R20 | 29.3" | 744 mm | 20 mm |
295/45R18 | 28.5" | 723 mm | 18 mm |
295/50R16 | 27.6" | 702 mm | 16 mm |
295/55R15 | 27.8" | 705 mm | 15 mm |
295/60R15 | 28.9" | 735 mm | 15 mm |
305/25R22 | 28" | 711 mm | 22 mm |
305/25R23 | 29" | 736 mm | 23 mm |
305/30R21 | 28.2" | 717 mm | 21 mm |
305/30R22 | 29.3" | 743 mm | 22 mm |
305/35R20 | 28.4" | 722 mm | 20 mm |
305/45R17 | 27.8" | 706 mm | 17 mm |
305/45R18 | 28.8" | 731 mm | 18 mm |
315/25R22 | 28.2" | 717 mm | 22 mm |
315/25R23 | 29.2" | 742 mm | 23 mm |
315/30R21 | 28.5" | 723 mm | 21 mm |
315/35R19 | 27.7" | 703 mm | 19 mm |
315/35R20 | 28.7" | 728 mm | 20 mm |
315/40R18 | 27.9" | 709 mm | 18 mm |
315/40R19 | 28.9" | 735 mm | 19 mm |
315/45R17 | 28.2" | 716 mm | 17 mm |
325/30R20 | 27.7" | 704 mm | 20 mm |
325/30R21 | 28.7" | 729 mm | 21 mm |
325/35R20 | 29" | 736 mm | 20 mm |
325/50R15 | 27.8" | 707 mm | 15 mm |
335/25R22 | 28.6" | 727 mm | 22 mm |
335/30R20 | 28" | 710 mm | 20 mm |
345/30R20 | 28.2" | 716 mm | 20 mm |
345/35R19 | 28.5" | 725 mm | 19 mm |
345/40R17 | 27.9" | 708 mm | 17 mm |
355/25R21 | 28" | 711 mm | 21 mm |