255/50R18 vs 255/40R20
255/40R20 is 0.01 in (0.2 mm) shorter than 255/50R18, a −0.03% diameter difference. With the speedometer set for 255/50R18, an indicated 60 mph is 60.0 mph on 255/40R20.
Is this a safe swap?
Diameter difference −0.03%: under 1%
By overall diameter alone, 255/40R20 is a near match for 255/50R18; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.
| Measurement | 255/50R18 | 255/40R20 | Difference | % diff |
|---|---|---|---|---|
| Overall diameter | 28.04 in (712.2 mm) | 28.03 in (712.0 mm) | −0.01 in (−0.2 mm) | −0.03% |
| Sidewall height | 5.02 in (127.5 mm) | 4.02 in (102.0 mm) | −1.00 in (−25.5 mm) | −20.00% |
| Section width | 10.04 in (255.0 mm) | 10.04 in (255.0 mm) | 0.00 in (0.0 mm) | 0.00% |
| Circumference | 88.09 in (2,237.4 mm) | 88.06 in (2,236.8 mm) | −0.02 in (−0.6 mm) | −0.03% |
| Revolutions per mile | 719 | 719 | 0 | +0.03% |
| Indicated | Actual on 255/50R18 | Actual on 255/40R20 | Difference |
|---|---|---|---|
| 30 mph | 30.0 mph | 30.0 mph | 0.0 mph |
| 60 mph | 60.0 mph | 60.0 mph | 0.0 mph |
| 75 mph | 75.0 mph | 75.0 mph | 0.0 mph |
Ride-height change: 255/40R20 lowers the vehicle by 0.00 in (0.1 mm) at the axle (half the −0.2 mm diameter difference).
255/40R20 is 0.01 in shorter than 255/50R18 and the same width, a 0.03% difference in overall diameter.
With the speedometer calibrated for 255/50R18, an indicated 60 mph is actually 60.0 mph on 255/40R20, and revolutions per mile change from 719 to 719.
255/40R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 255/50R18.
Size pages
Other comparisons with 255/50R18
- 215/70R16 vs 255/50R18
- 235/65R16 vs 255/50R18
- 205/75R16 vs 255/50R18
- 225/70R16 vs 255/50R18
- 215/65R17 vs 255/50R18
- 235/60R17 vs 255/50R18
- 255/45R18 vs 255/50R18
- 235/50R18 vs 255/50R18
Other comparisons with 255/40R20
- 285/45R18 vs 255/40R20
- 235/55R18 vs 255/40R20
- 225/50R19 vs 255/40R20
- 285/40R19 vs 255/40R20
- 255/45R19 vs 255/40R20
- 235/50R19 vs 255/40R20
- 255/35R20 vs 255/40R20
- 245/40R20 vs 255/40R20
Source: Calculated from standard tire geometry formulas. Verified . How these numbers are calculated.