21-inch rim · tyre size comparison
275/45 R21 vs 315/40 R21
315/40 R21 is 4.5 mm (0.18 in) taller than 275/45 R21, a +0.58% difference in overall diameter. That is well inside the ±3% limit commonly used to screen a tire size change; it is a nominal calculation, not a fitment approval.
- Diameter difference
- +4.5 mm
- +0.18 in · +0.58%
- Indicated 70 mph
- 70.4 mph
- Actual speed on 315/40 R21, if the speedometer was accurate on 275/45 R21.
- Sidewall height
- +2.25 mm
- 123.75 mm to 126 mm.
Dimensions side by side
| Measurement | 275/45 R21 | 315/40 R21 | Difference |
|---|---|---|---|
| Overall diameter | 780.9 mm30.74 in | 785.4 mm30.92 in | +4.5 mm (+0.58%)+0.18 in |
| Section width | 275 mm10.83 in | 315 mm12.4 in | +40 mm+1.57 in |
| Sidewall height | 123.75 mm4.87 in | 126 mm4.96 in | +2.25 mm+0.09 in |
| Circumference | 2,453.27 mm96.59 in | 2,467.41 mm97.14 in | +14.14 mm+0.56 in |
| Rim diameter | 533.4 mm21 in | 533.4 mm21 in | 0 mm0 in |
| Revolutions per mile | 656 | 652.24 | -3.76 |
| Revolutions per km | 407.62 | 405.28 | -2.34 |
Calculated from the nominal size codes, without load or tread wear. Real tyres vary by model, rim width and pressure, so the exact tyre's published dimensions take priority.
Speedometer with 315/40 R21
If the speedometer was accurate on 275/45 R21, fitting 315/40 R21 means the car is travelling 0.58% faster than it shows, and the odometer under-records distance by about the same amount.
| Speedometer shows | Actual on 315/40 R21 | Difference |
|---|---|---|
| 30 mph | 30.17 mph | +0.17 mph |
| 50 mph | 50.29 mph | +0.29 mph |
| 60 mph | 60.35 mph | +0.35 mph |
| 70 mph | 70.4 mph | +0.4 mph |
| 50 km/h | 50.29 km/h | +0.29 km/h |
| 100 km/h | 100.58 km/h | +0.58 km/h |
UN ECE Regulation 39, used in the UK and EU, does not allow a speedometer to show less than the true speed, so many read slightly high from the factory. A taller tyre uses up some of that margin. Calibration varies by car; these figures are the tyre-size effect only.
What changes from 275/45 R21 to 315/40 R21
Wheels
Both fit 21-inch wheels. Our rim-width starting estimate is 8.5 to 9.5 inches for 275/45 R21 and 10 to 11 inches for 315/40 R21, so the two may not suit the same wheel width. The exact tyre's approved rim range takes priority.
Sidewall and ride comfort
315/40 R21 has 2.25 mm more sidewall (126 vs 123.75 mm). A taller sidewall usually rides a little softer and gives the rim more protection from potholes and kerbs, at the cost of some steering sharpness.
Contact width
315/40 R21 is 40 mm wider in nominal section width (14.5%). A wider tyre usually puts a wider, shorter contact patch on the road, which can help dry grip, but it may follow ruts more, clear standing water less readily and add some rolling resistance.
Wheel well clearance
Ride height changes by about +2.3 mm, and the top of the tyre moves the same distance closer to the arch. On the same wheel width and offset, each sidewall moves about 20 mm further out towards the arch lip and further in towards the suspension. Check clearance at full steering lock and full suspension compression, especially on a lowered car.
Gearing
The effective final drive changes by -0.57%: 3,000 rpm at a given road speed becomes about 2,980 rpm. Gearing gets slightly longer, so cruising revs drop and acceleration in each gear softens.
ABS, stability control and AWD
These systems compare wheel speeds. Fit the same size on both ends of an axle, and follow the car maker's rolling-circumference limits, which can be much tighter than 3%.
General tendencies from the nominal numbers, not a test of any tyre. Load index, speed rating, tyre construction and the car's approved sizes matter more than these rules of thumb.
Explained in our guides
- 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.
- Staggered vs square wheels: tyres, rotation and AWDCompare square and staggered wheel setups, calculate front-to-rear tyre diameter, and understand rotation limits and all-wheel-drive requirements.
- 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.
Cars in our guides using these sizes
Factory trims and popular community setups from our fitment guides. A popular setup is not a factory approval; each guide explains its trims, axles and clearance limits.
275/45 R21
- Land Rover Range Rover L405 (2012–2021)Factory trim
- Land Rover Range Rover Sport L494 (2013–2022)Factory trim
315/40 R21
- Mercedes-Benz G63 AMG W463A (2018–2024)Popular setup
More comparisons
275/45 R21 compared
Questions
Can I use 315/40 R21 instead of 275/45 R21?
Both fit 21-inch wheels. The diameter difference is +0.58%, well inside the ±3% screening limit. That does not approve fitment: check the car maker's approved sizes, load index, speed rating, rim width and clearance first.
How much bigger is 315/40 R21 than 275/45 R21?
Overall diameter is 4.5 mm (0.18 in) larger: 785.4 vs 780.9 mm. Section width is 40 mm wider, and the sidewall is 2.25 mm taller. These are nominal figures; real tyres vary by model and rim width.
Will 315/40 R21 change my speedometer reading?
Yes, by 0.58%. If the speedometer was accurate on 275/45 R21, an indicated 60 mph would be about 60.3 mph on 315/40 R21, and an indicated 100 km/h about 100.6 km/h. You would be going faster than the speedometer shows.
Which is better, 275/45 R21 or 315/40 R21?
Neither in general. 315/40 R21 has the taller sidewall, which usually rides more comfortably; 275/45 R21 usually steers more sharply. 315/40 R21 is wider, which can help dry grip; the narrower size is usually better in standing water and snow. The size listed for your car's trim, on its placard or in the handbook, is the safe default.
Check the exact tyre
Size codes give nominal dimensions. Before buying, confirm the size is approved for your car and check the exact tyre's load and speed ratings and published dimensions.