20-inch rim · tyre size comparison
235/35 R20 vs 245/30 R20
245/30 R20 is 17.5 mm (0.69 in) shorter than 235/35 R20, a -2.6% difference in overall diameter. That is inside the ±3% limit commonly used to screen a tire size change; it is a nominal calculation, not a fitment approval.
- Diameter difference
- -17.5 mm
- -0.69 in · -2.6%
- Indicated 70 mph
- 68.2 mph
- Actual speed on 245/30 R20, if the speedometer was accurate on 235/35 R20.
- Sidewall height
- -8.75 mm
- 82.25 mm to 73.5 mm.
Dimensions side by side
| Measurement | 235/35 R20 | 245/30 R20 | Difference |
|---|---|---|---|
| Overall diameter | 672.5 mm26.48 in | 655 mm25.79 in | -17.5 mm (-2.6%)-0.69 in |
| Section width | 235 mm9.25 in | 245 mm9.65 in | +10 mm+0.39 in |
| Sidewall height | 82.25 mm3.24 in | 73.5 mm2.89 in | -8.75 mm-0.34 in |
| Circumference | 2,112.72 mm83.18 in | 2,057.74 mm81.01 in | -54.98 mm-2.16 in |
| Rim diameter | 508 mm20 in | 508 mm20 in | 0 mm0 in |
| Revolutions per mile | 761.74 | 782.09 | +20.35 |
| Revolutions per km | 473.32 | 485.97 | +12.65 |
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 245/30 R20
If the speedometer was accurate on 235/35 R20, fitting 245/30 R20 means the car is travelling 2.6% slower than it shows, and the odometer over-records distance by about the same amount.
| Speedometer shows | Actual on 245/30 R20 | Difference |
|---|---|---|
| 30 mph | 29.22 mph | -0.78 mph |
| 50 mph | 48.7 mph | -1.3 mph |
| 60 mph | 58.44 mph | -1.56 mph |
| 70 mph | 68.18 mph | -1.82 mph |
| 50 km/h | 48.7 km/h | -1.3 km/h |
| 100 km/h | 97.4 km/h | -2.6 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 235/35 R20 to 245/30 R20
Wheels
Both fit 20-inch wheels. Our rim-width starting estimate is 7.5 to 8.5 inches for 235/35 R20 and 7.5 to 8.5 inches for 245/30 R20, so a 7.5 to 8.5-inch wheel sits in both ranges. The exact tyre's approved rim range takes priority.
Sidewall and ride comfort
245/30 R20 has 8.75 mm less sidewall (73.5 vs 82.25 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
245/30 R20 is 10 mm wider in nominal section width (4.3%). 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 -8.8 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 +2.67%: 3,000 rpm at a given road speed becomes about 3,080 rpm. Gearing gets slightly 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 2.6% diameter gap, never mix 235/35 R20 and 245/30 R20 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.
235/35 R20
- Porsche 718 Cayman 982 (2016–present)Factory trim
- Tesla Model 3 Original / Highland (2017–present)Factory trim
245/30 R20
- Audi R8 Type 4S (2015–2024)Factory trim
- Honda Civic Type R FK8 (2017–2021)Factory trim
- Mercedes-Benz E63 AMG W212 / S212 (2009–2016)Popular setup
More comparisons
235/35 R20 compared
Questions
Can I use 245/30 R20 instead of 235/35 R20?
Both fit 20-inch wheels. The diameter difference is -2.6%, 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 245/30 R20 than 235/35 R20?
Overall diameter is 17.5 mm (0.69 in) smaller: 655 vs 672.5 mm. Section width is 10 mm wider, and the sidewall is 8.75 mm shorter. These are nominal figures; real tyres vary by model and rim width.
Will 245/30 R20 change my speedometer reading?
Yes, by 2.6%. If the speedometer was accurate on 235/35 R20, an indicated 60 mph would be about 58.4 mph on 245/30 R20, and an indicated 100 km/h about 97.4 km/h. You would be going slower than the speedometer shows.
Which is better, 235/35 R20 or 245/30 R20?
Neither in general. 235/35 R20 has the taller sidewall, which usually rides more comfortably; 245/30 R20 usually steers more sharply. 245/30 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.