20-inch vs 21-inch rims · tyre size comparison
255/45 R20 vs 255/40 R21
255/40 R21 and 255/45 R20 are effectively the same height: 255/40 R21 is 0.1 mm shorter in overall diameter, a -0.01% difference. 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
- -0.1 mm
- 0 in · -0.01%
- Indicated 70 mph
- 70 mph
- Actual speed on 255/40 R21, if the speedometer was accurate on 255/45 R20.
- Sidewall height
- -12.75 mm
- 114.75 mm to 102 mm.
Dimensions side by side
| Measurement | 255/45 R20 | 255/40 R21 | Difference |
|---|---|---|---|
| Overall diameter | 737.5 mm29.04 in | 737.4 mm29.03 in | -0.1 mm (-0.01%)0 in |
| Section width | 255 mm10.04 in | 255 mm10.04 in | 0 mm0 in |
| Sidewall height | 114.75 mm4.52 in | 102 mm4.02 in | -12.75 mm-0.5 in |
| Circumference | 2,316.92 mm91.22 in | 2,316.61 mm91.21 in | -0.31 mm-0.01 in |
| Rim diameter | 508 mm20 in | 533.4 mm21 in | +25.4 mm+1 in |
| Revolutions per mile | 694.6 | 694.7 | +0.09 |
| Revolutions per km | 431.61 | 431.67 | +0.06 |
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 255/40 R21
The nominal diameters match, so swapping 255/45 R20 for 255/40 R21 has no speedometer effect.
| Speedometer shows | Actual on 255/40 R21 | Difference |
|---|---|---|
| 30 mph | 30 mph | 0 mph |
| 50 mph | 49.99 mph | -0.01 mph |
| 60 mph | 59.99 mph | -0.01 mph |
| 70 mph | 69.99 mph | -0.01 mph |
| 50 km/h | 49.99 km/h | -0.01 km/h |
| 100 km/h | 99.99 km/h | -0.01 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 255/45 R20 to 255/40 R21
Wheels
These sizes need different wheels: 20-inch for 255/45 R20 and 21-inch for 255/40 R21. The larger wheel usually weighs a little more; the smaller one must still clear the brake calipers. Check rim width, offset and load rating for whichever wheel you fit.
Sidewall and ride comfort
255/40 R21 has 12.75 mm less sidewall (102 vs 114.75 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
Both are 255 mm in nominal section width, so the tread is about as wide. Contact patch area is set mostly by load and pressure, so grip differences come more from the tyre model than this size change.
Wheel well clearance
Ride height is effectively unchanged. On the same wheel, the sidewalls stay in roughly the same place side to side. Nominal clearance is unchanged, though real tyre models can differ by a few millimetres.
Gearing
Effective gearing changes by +0.01%, too little to notice from the driver's seat.
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.
255/45 R20
- Hyundai IONIQ 5 / IONIQ 5 N NE (2021–present)Factory trim
- Kia EV6 / EV6 GT CV (2021–present)Factory trim
255/40 R21
- Kia EV6 / EV6 GT CV (2021–present)Factory trim
More comparisons
255/45 R20 compared
Questions
Can I use 255/40 R21 instead of 255/45 R20?
Only on 21-inch wheels, not the 20-inch wheels that take 255/45 R20. The diameter difference is -0.01%, 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 255/40 R21 than 255/45 R20?
Overall diameter is 0.1 mm (0 in) smaller: 737.4 vs 737.5 mm. Section width is the same, and the sidewall is 12.75 mm shorter. These are nominal figures; real tyres vary by model and rim width.
Will 255/40 R21 change my speedometer reading?
No. The nominal diameter is the same, so the speedometer effect is nil.
Which is better, 255/45 R20 or 255/40 R21?
Neither in general. 255/45 R20 has the taller sidewall, which usually rides more comfortably; 255/40 R21 usually steers more sharply. 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.