21-inch rim · dimensions and fitment reference
315/40 R21 tyre size
315/40 R21 has a nominal diameter of 785.4 mm (30.92 inches) and a 126 mm sidewall. Check the dimensions, car-guide matches and neighbouring rim-size alternatives below.
315/40 R21 dimensions
| Measurement | Millimetres | Inches |
|---|---|---|
| Overall diameter | 785.4 | 30.92 |
| Sidewall height | 126 | 4.96 |
| Section width | 315 | 12.4 |
| Circumference | 2,467.41 | 97.14 |
| Rim diameter | 533.4 | 21 |
- Revolutions per kilometre
- 405.28
- Revolutions per mile
- 652.24
Calculated from the nominal size, without load or tread wear. Revolutions use nominal circumference, not a measured rolling circumference. Tyre model, inflation, load and rim width can change the real dimensions.
Reading the size
315 is the nominal section width in millimetres, not tread width. 40 is the sidewall height as a percentage of that width. R means radial construction, and 21 is the rim diameter in inches. For the load index, speed rating and date code that follow the size, read how to read a tyre size.
Rim-width starting range
10 to 11 inches
Our passenger-car guidance estimates a 10.5-inch measuring rim and a half-inch band either side. This is a starting estimate, not an approved rim-width range. The exact tyre manufacturer's published range takes priority. Our guide to choosing wheel width for a tyre explains how to find the approved range.
Explore tyre and rim widthExplained in our guides
- How to read a tyre size, load index and sidewall markingsDecode tyre width, aspect ratio, rim diameter, load and speed markings, then calculate sidewall height and diameter using a 225/45 R17 example.
- Plus-sizing wheels: tyre diameter and speedometer changesWork through a 17 to 18 inch wheel change, compare tyre diameters and speedometer effects, and understand why a 3% rule cannot approve fitment.
- How to choose wheel width for a tyre: use the approved rangeChoose a rim width using the exact tyre manufacturer's data, distinguish measuring rims from approved ranges, and check the effect on inner and outer clearance.
Plus-size and minus-size alternatives
Computed candidates within ±3% of the 785.4 mm diameter, on 20-inch and 22-inch rims. Width stays within 20 mm of this tyre, using 165 to 355 mm widths in 10 mm steps and 25 to 80% profiles in 5% steps. These combinations are not a stock list or fitment approval; check availability and the exact tyre specification.
Minus one: 20-inch rims
| Candidate size | Diameter | Change | Compare |
|---|---|---|---|
| 305/45 R20 | 782.5 mm | -0.37% | Compare sizes |
| 315/45 R20 | 791.5 mm | +0.78% | Compare sizes |
| 335/40 R20 | 776 mm | -1.2% | Compare sizes |
| 295/45 R20 | 773.5 mm | -1.52% | Compare sizes |
| 325/45 R20 | 800.5 mm | +1.92% | Compare sizes |
| 325/40 R20 | 768 mm | -2.22% | Compare sizes |
| 295/50 R20 | 803 mm | +2.24% | Compare sizes |
Plus one: 22-inch rims
| Candidate size | Diameter | Change | Compare |
|---|---|---|---|
| 325/35 R22 | 786.3 mm | +0.11% | Compare sizes |
| 315/35 R22 | 779.3 mm | -0.78% | Compare sizes |
| 335/35 R22 | 793.3 mm | +1.01% | Compare sizes |
| 295/40 R22 | 794.8 mm | +1.2% | Compare sizes |
| 305/35 R22 | 772.3 mm | -1.67% | Compare sizes |
| 305/40 R22 | 802.8 mm | +2.22% | Compare sizes |
| 295/35 R22 | 765.3 mm | -2.56% | Compare sizes |
How a size change moves the speedometer and gearing is covered in plus-sizing wheels. A smaller rim must still clear the brakes. Check wheel width, offset, load and speed ratings, full steering and suspension travel, and vehicle restrictions. AWD and staggered cars may need a much tighter rolling-circumference match than this screening limit.
Cars and setups listing 315/40 R21
Use the trim, market and axle as well as the car name. Each guide links to its supporting sources and explains clearance limits.
Factory trims
No factory trims in our guides list this size.
Popular community setups
| Car | Setup | Axle |
|---|---|---|
| Mercedes-Benz G63 AMG W463A (2018–2024) | Factory 21-inch with wider tyres | Front and rear |
315/40 R21 compared with
Sizes people often weigh against 315/40 R21, with the full dimension, speedometer and gearing comparison on each page. The percentage is the change in overall diameter from 315/40 R21.
All tyre size comparisonsQuestions
How tall is a 315/40 R21 tyre?
Its nominal overall diameter is 785.4 mm (30.92 inches). Sidewall height is 126 mm. These are size-code calculations; actual dimensions vary by tyre model, rim and operating conditions.
What rim width should I use for 315/40 R21?
Our starting estimate is 10 to 11 inches, around a 10.5-inch measuring rim. This is not a manufacturer-approved range. Check the exact tyre model's published rim widths before fitting.
Can I replace 315/40 R21 with one of the alternatives?
The alternatives only match nominal diameter within 3% on a rim one inch smaller or larger. That does not approve fitment. Confirm that the size is manufactured, then check load and speed ratings, rim width, brakes, bodywork clearance, vehicle approval and any AWD requirements.
Which cars use 315/40 R21?
1 car guides list this size. The tables separate factory trims from community setups and identify the axle. A popular setup is not a factory tyre approval.
Check the exact tyre
For how measured dimensions and approved rim ranges differ from nominal calculations, see Tire Rack’s tyre specification guide.