315/40R21 in inches & centimeters
315/40R21 has a nominal diameter of 30.92 inches and width of 12.40 inches, with a 4.96-inch sidewall on a 21-inch wheel. Nominal tire dimensions calculated directly from the size code. Use these numbers to compare geometry before checking the actual tire specification.
≈ 30.92 × 12.40R21 in — nominal inch notation. Check the tire maker’s catalog for available product sizes.
This reference is available in English. ES / PT open the translated calculator with this size.
| Measurement | inches | centimeters | millimeters |
|---|---|---|---|
| Overall diameter | 30.92 in | 78.54 cm | 785.4 mm |
| Section width | 12.40 in | 31.50 cm | 315.0 mm |
| Sidewall height | 4.96 in | 12.60 cm | 126.0 mm |
| Circumference | 97.14 in | 246.74 cm | 2,467.4 mm |
| Wheel diameter | 21.00 in | 53.34 cm | 533.4 mm |
| Revolutions / mi | 652.2 | ||
| Revolutions / km | 405.3 | ||
Compare this size
Nominal dimensions · 315/40R21
Side view
DOverall diameter30.92 in78.54 cm
SSidewall height4.96 in12.60 cm
WDWheel diameter21.00 in53.34 cm
Front view
DOverall diameter30.92 in78.54 cm
WSection width12.40 in31.50 cm
- DOverall diameter
- 30.92 in78.54 cm
- WSection width
- 12.40 in31.50 cm
- SSidewall height
- 4.96 in12.60 cm
- WDWheel diameter
- 21.00 in53.34 cm
- CCircumference
- 97.14 in246.74 cm
Approximate inch notation≈ 30.92 × 12.40R21
Diameter × width, with wheel diameter in inches, derived from the nominal size code. Use the tire maker’s catalog to choose an available product size.
Add a second size to compare dimensions and speedometer change.
Reference model marked 245/40 ZR18 97Y.
Product image: Continental Tire
Tread-detail reference. Section width is calculated separately.
Product image: Continental TireHow we calculate it
315 = section width in millimeters; 40 = sidewall as a percentage of width; 21 = wheel diameter in inches.
Sidewall height: 315 × 40/100 = 126 mm
Overall diameter: (21 × 25.4) + (2 × 126) = 785.4 mm
Circumference: π × 785.4 ≈ 2,467.4 mm
Read the size-code guide Overall diameter: (21 × 25.4) + (2 × 126) = 785.4 mm
Circumference: π × 785.4 ≈ 2,467.4 mm
What to check before changing size
Use the vehicle and tire specifications to check these three points:
- Vehicle placard and manual: match an approved size and the required load and speed ratings.
- Wheel specifications: check the tire maker’s wheel-width range and the wheel offset.
- Available space: check clearance at full steering lock and throughout suspension travel.
Method / data
Dimensions are calculated from the nominal size code. The reference catalog lists size codes; the tire maker’s specification sheet gives the exact model’s published measurements.
Calculation method and reference dataNearby sizes to explore
Sorted by nominal diameter difference. Compare the tire maker’s published dimensions for the sizes you shortlist.