20-inch rim · tyre size comparison
245/45 R20 vs 255/40 R20
255/40 R20 is 16.5 mm (0.65 in) shorter than 245/45 R20, a -2.26% 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
- -16.5 mm
- -0.65 in · -2.26%
- Indicated 70 mph
- 68.4 mph
- Actual speed on 255/40 R20, if the speedometer was accurate on 245/45 R20.
- Sidewall height
- -8.25 mm
- 110.25 mm to 102 mm.
Dimensions side by side
| Measurement | 245/45 R20 | 255/40 R20 | Difference |
|---|---|---|---|
| Overall diameter | 728.5 mm28.68 in | 712 mm28.03 in | -16.5 mm (-2.26%)-0.65 in |
| Section width | 245 mm9.65 in | 255 mm10.04 in | +10 mm+0.39 in |
| Sidewall height | 110.25 mm4.34 in | 102 mm4.02 in | -8.25 mm-0.32 in |
| Circumference | 2,288.65 mm90.1 in | 2,236.81 mm88.06 in | -51.84 mm-2.04 in |
| Rim diameter | 508 mm20 in | 508 mm20 in | 0 mm0 in |
| Revolutions per mile | 703.18 | 719.48 | +16.3 |
| Revolutions per km | 436.94 | 447.06 | +10.13 |
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 R20
If the speedometer was accurate on 245/45 R20, fitting 255/40 R20 means the car is travelling 2.26% slower than it shows, and the odometer over-records distance by about the same amount.
| Speedometer shows | Actual on 255/40 R20 | Difference |
|---|---|---|
| 30 mph | 29.32 mph | -0.68 mph |
| 50 mph | 48.87 mph | -1.13 mph |
| 60 mph | 58.64 mph | -1.36 mph |
| 70 mph | 68.41 mph | -1.59 mph |
| 50 km/h | 48.87 km/h | -1.13 km/h |
| 100 km/h | 97.74 km/h | -2.26 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 245/45 R20 to 255/40 R20
Wheels
Both fit 20-inch wheels. Our rim-width starting estimate is 7.5 to 8.5 inches for 245/45 R20 and 8 to 9 inches for 255/40 R20, so a 8 to 8.5-inch wheel sits in both ranges. The exact tyre's approved rim range takes priority.
Sidewall and ride comfort
255/40 R20 has 8.25 mm less sidewall (102 vs 110.25 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
255/40 R20 is 10 mm wider in nominal section width (4.1%). 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 -8.3 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 +2.32%: 3,000 rpm at a given road speed becomes about 3,070 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 2.3% diameter gap, never mix 245/45 R20 and 255/40 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.
245/45 R20
- Chevrolet Camaro 5th gen (2010–2015)Factory trim
- Dodge Challenger LC (2008–2023)Factory trim
- Dodge Charger LD (2011–2023)Factory trim
255/40 R20
- Nissan GT-R R35 (2007–2025)Factory trim
- Tesla Model Y 1st gen, pre-Juniper (2020–2025)Factory and popular setup
More comparisons
245/45 R20 compared
Questions
Can I use 255/40 R20 instead of 245/45 R20?
Both fit 20-inch wheels. The diameter difference is -2.26%, 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 R20 than 245/45 R20?
Overall diameter is 16.5 mm (0.65 in) smaller: 712 vs 728.5 mm. Section width is 10 mm wider, and the sidewall is 8.25 mm shorter. These are nominal figures; real tyres vary by model and rim width.
Will 255/40 R20 change my speedometer reading?
Yes, by 2.26%. If the speedometer was accurate on 245/45 R20, an indicated 60 mph would be about 58.6 mph on 255/40 R20, and an indicated 100 km/h about 97.7 km/h. You would be going slower than the speedometer shows.
Which is better, 245/45 R20 or 255/40 R20?
Neither in general. 245/45 R20 has the taller sidewall, which usually rides more comfortably; 255/40 R20 usually steers more sharply. 255/40 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.