18-inch rim · tyre size comparison
305/35 R18 vs 315/30 R18
315/30 R18 is 24.5 mm (0.96 in) shorter than 305/35 R18, a -3.65% difference in overall diameter. That is just outside the ±3% limit commonly used to screen a tire size change, so expect a noticeable speedometer error; it is a nominal calculation, not a fitment approval.
- Diameter difference
- -24.5 mm
- -0.96 in · -3.65%
- Indicated 70 mph
- 67.4 mph
- Actual speed on 315/30 R18, if the speedometer was accurate on 305/35 R18.
- Sidewall height
- -12.25 mm
- 106.75 mm to 94.5 mm.
Dimensions side by side
| Measurement | 305/35 R18 | 315/30 R18 | Difference |
|---|---|---|---|
| Overall diameter | 670.7 mm26.41 in | 646.2 mm25.44 in | -24.5 mm (-3.65%)-0.96 in |
| Section width | 305 mm12.01 in | 315 mm12.4 in | +10 mm+0.39 in |
| Sidewall height | 106.75 mm4.2 in | 94.5 mm3.72 in | -12.25 mm-0.48 in |
| Circumference | 2,107.07 mm82.96 in | 2,030.1 mm79.93 in | -76.97 mm-3.03 in |
| Rim diameter | 457.2 mm18 in | 457.2 mm18 in | 0 mm0 in |
| Revolutions per mile | 763.78 | 792.74 | +28.96 |
| Revolutions per km | 474.59 | 492.59 | +17.99 |
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 R18
If the speedometer was accurate on 305/35 R18, fitting 315/30 R18 means the car is travelling 3.65% slower than it shows, and the odometer over-records distance by about the same amount.
| Speedometer shows | Actual on 315/30 R18 | Difference |
|---|---|---|
| 30 mph | 28.9 mph | -1.1 mph |
| 50 mph | 48.17 mph | -1.83 mph |
| 60 mph | 57.81 mph | -2.19 mph |
| 70 mph | 67.44 mph | -2.56 mph |
| 50 km/h | 48.17 km/h | -1.83 km/h |
| 100 km/h | 96.35 km/h | -3.65 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 shorter tyre adds to it. Calibration varies by car; these figures are the tyre-size effect only.
What changes from 305/35 R18 to 315/30 R18
Wheels
Both fit 18-inch wheels. Our rim-width starting estimate is 9.5 to 10.5 inches for 305/35 R18 and 10 to 11 inches for 315/30 R18, 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 R18 has 12.25 mm less sidewall (94.5 vs 106.75 mm). A shorter sidewall usually sharpens steering response, but rides a little firmer and leaves the rim more exposed to potholes and kerbs.
Contact width
315/30 R18 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 -12.3 mm, and the top of the tyre moves the same distance away from 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 +3.79%: 3,000 rpm at a given road speed becomes about 3,110 rpm. Gearing gets noticeably shorter, so acceleration in each gear sharpens and cruising revs rise.
ABS, stability control and AWD
These systems compare wheel speeds. Fit the same size on both ends of an axle; with a 3.7% diameter gap, never mix 305/35 R18 and 315/30 R18 on an AWD car, 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.
305/35 R18
- BMW M4 F82 (2014–2020)Popular setup
- Chevrolet Corvette C5 (1997–2004)Popular setup
- Porsche 911 997 (2004–2012)Popular setup
315/30 R18
- Chevrolet Corvette C5 (1997–2004)Popular setup
Questions
Can I use 315/30 R18 instead of 305/35 R18?
Both fit 18-inch wheels. The diameter difference is -3.65%, just outside the ±3% screening limit, so most fitters would treat it as a different size class rather than a direct replacement. 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 R18 than 305/35 R18?
Overall diameter is 24.5 mm (0.96 in) smaller: 646.2 vs 670.7 mm. Section width is 10 mm wider, and the sidewall is 12.25 mm shorter. These are nominal figures; real tyres vary by model and rim width.
Will 315/30 R18 change my speedometer reading?
Yes, by 3.65%. If the speedometer was accurate on 305/35 R18, an indicated 60 mph would be about 57.8 mph on 315/30 R18, and an indicated 100 km/h about 96.3 km/h. You would be going slower than the speedometer shows.
Which is better, 305/35 R18 or 315/30 R18?
Neither in general. 305/35 R18 has the taller sidewall, which usually rides more comfortably; 315/30 R18 usually steers more sharply. 315/30 R18 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.