315/40R21 refers to the size of the tire. It’s a standard which is used by car and tire manufacturers. By having the standard, tire manufacturers are able to manufacture same size tires independently and users can buy same size tires from different manufacturers.
Here’s an explanation of what these metrics mean in practice:
| 315 | The tire width is 315mm. |
| 40 | The aspect ratio of the tire is 40. Aspect ratio refers to the height of tire sidewall in relation to tire width in percentages. |
| R | The letter in the middle refers to tire construction. Practically all tires sold for cars are radials. Sometimes you also see tires with RF or ZR. RF indicates the tire is run flat, whereas ZR means the tire has speed rating higher than V. These tires are also radials, as the R indicates. |
| 21 | Rim diameter for the tire. 315/40R21 tire can be mounted only to a 21″ rims. |
Best selling 315/40R21 tires
MICHELIN Primacy Tour A/S, All-Season Car Tire, Sport and Performance Cars - 315/40R21 111H
(as of May 31, 2023 16:20 GMT +00:00 - More infoProduct prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on [relevant Amazon Site(s), as applicable] at the time of purchase will apply to the purchase of this product.)
315/40R21 111Y PIRELLI P ZERO(PZ4-SPT) BW (MO)
6% Off(as of May 31, 2023 16:20 GMT +00:00 - More infoProduct prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on [relevant Amazon Site(s), as applicable] at the time of purchase will apply to the purchase of this product.)
Continental CrossContact LX Sport All-Season Radial Tire -315/40R21 111H
(as of May 31, 2023 16:20 GMT +00:00 - More infoProduct prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on [relevant Amazon Site(s), as applicable] at the time of purchase will apply to the purchase of this product.)
What is 315/40R21 tire size in inches?
Because the width of 315/40R21 tire is stated in millimeters and sidewall height in percentages, you have to make calculations to see what the true size of 315/40R21 is in inches. Luckily, we have made these calculations for you.In this table you can see size of 315/40R21 in inches, as well as few additional metrics:
| In inches | In millimeters | |
| Section Width | 12.4″ | 315mm |
| Sidewall height | 5.0″ | 126mm |
| Diameter / Height | 30.9″ | 785mm |
| Circumference | 97.1″ | 2467mm |
| Rim Diameter | 21″ | |
| Revolutions | 652 per mile | 405 per kilometer |
Recommended Rim Sizes for 315/40R21 Tires
For 315/40R21 tires you need to have correct size rims to install the tires to. There are different width rims available in the market, and you can run into problems by installing 315/40R21 to a too wide or too narrow rim. Here are the recommended rim sizes for 315/40R21 tire:
| Diameter | Width Range |
| 21″ | 10.5-12.5″ |
315/40R21 – Variations
When you look at the tire code on 315/40R21 tires, you may see there are additional numbers and letters after the code 315/40R21. These numbers and letters refer to speed rating and load index of the tire. 315/40R21 has at least following variations available on the markets:
315/40R21 111H 315/40R21 111V 315/40R21 111Y 315/40R21 115V