20-inch rim · tyre size comparison
305/30 R20 vs 315/30 R20
315/30 R20 is 6 mm (0.24 in) taller than 305/30 R20, a +0.87% 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
- +6 mm
- +0.24 in · +0.87%
- Indicated 70 mph
- 70.6 mph
- Actual speed on 315/30 R20, if the speedometer was accurate on 305/30 R20.
- Sidewall height
- +3 mm
- 91.5 mm to 94.5 mm.
Dimensions side by side
| Measurement | 305/30 R20 | 315/30 R20 | Difference |
|---|---|---|---|
| Overall diameter | 691 mm27.2 in | 697 mm27.44 in | +6 mm (+0.87%)+0.24 in |
| Section width | 305 mm12.01 in | 315 mm12.4 in | +10 mm+0.39 in |
| Sidewall height | 91.5 mm3.6 in | 94.5 mm3.72 in | +3 mm+0.12 in |
| Circumference | 2,170.84 mm85.47 in | 2,189.69 mm86.21 in | +18.85 mm+0.74 in |
| Rim diameter | 508 mm20 in | 508 mm20 in | 0 mm0 in |
| Revolutions per mile | 741.35 | 734.96 | -6.38 |
| Revolutions per km | 460.65 | 456.69 | -3.97 |
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 315/30 R20
If the speedometer was accurate on 305/30 R20, fitting 315/30 R20 means the car is travelling 0.87% faster than it shows, and the odometer under-records distance by about the same amount.
| Speedometer shows | Actual on 315/30 R20 | Difference |
|---|---|---|
| 30 mph | 30.26 mph | +0.26 mph |
| 50 mph | 50.43 mph | +0.43 mph |
| 60 mph | 60.52 mph | +0.52 mph |
| 70 mph | 70.61 mph | +0.61 mph |
| 50 km/h | 50.43 km/h | +0.43 km/h |
| 100 km/h | 100.87 km/h | +0.87 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 305/30 R20 to 315/30 R20
Wheels
Both fit 20-inch wheels. Our rim-width starting estimate is 9.5 to 10.5 inches for 305/30 R20 and 10 to 11 inches for 315/30 R20, so a 10 to 10.5-inch wheel sits in both ranges. The exact tyre's approved rim range takes priority.
Sidewall and ride comfort
315/30 R20 has 3 mm more sidewall (94.5 vs 91.5 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
315/30 R20 is 10 mm wider in nominal section width (3.3%). 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 +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 5 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 -0.86%: 3,000 rpm at a given road speed becomes about 2,970 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.
305/30 R20
- Audi R8 Type 4S (2015–2024)Factory and popular setup
- Chevrolet Camaro 6th gen (2016–2024)Factory and popular setup
- Chevrolet Corvette C8 (2020–present)Factory and popular setup
- Ford Mustang S650 (2024–present)Popular setup
- Mercedes-AMG AMG GT C190 / R190 (2014–2021)Factory trim
- Porsche 911 991 (2012–2019)Factory and popular setup
315/30 R20
- Porsche 911 992 (2019–present)Popular setup
More comparisons
305/30 R20 compared
315/30 R20 compared
Questions
Can I use 315/30 R20 instead of 305/30 R20?
Both fit 20-inch wheels. The diameter difference is +0.87%, 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 315/30 R20 than 305/30 R20?
Overall diameter is 6 mm (0.24 in) larger: 697 vs 691 mm. Section width is 10 mm wider, and the sidewall is 3 mm taller. These are nominal figures; real tyres vary by model and rim width.
Will 315/30 R20 change my speedometer reading?
Yes, by 0.87%. If the speedometer was accurate on 305/30 R20, an indicated 60 mph would be about 60.5 mph on 315/30 R20, and an indicated 100 km/h about 100.9 km/h. You would be going faster than the speedometer shows.
Which is better, 305/30 R20 or 315/30 R20?
Neither in general. 315/30 R20 has the taller sidewall, which usually rides more comfortably; 305/30 R20 usually steers more sharply. 315/30 R20 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.