TireSizesFinder

285/45R18 vs 255/40R20

255/40R20 is 0.07 in (1.7 mm) shorter than 285/45R18, a −0.24% diameter difference. With the speedometer set for 285/45R18, an indicated 60 mph is 59.9 mph on 255/40R20.

Is this a safe swap?

Diameter difference −0.24%: under 1%

By overall diameter alone, 255/40R20 is a near match for 285/45R18; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 285/45R18 (28.10 in) vs 255/40R20 (28.03 in)285/45R1828.10 in × 11.22 in255/40R2028.03 in × 10.04 in
Side profile and tread width, drawn to the same scale.
285/45R18 vs 255/40R20 differences
Measurement285/45R18255/40R20Difference% diff
Overall diameter28.10 in (713.7 mm)28.03 in (712.0 mm)−0.07 in (−1.7 mm)−0.24%
Sidewall height5.05 in (128.3 mm)4.02 in (102.0 mm)−1.03 in (−26.2 mm)−20.47%
Section width11.22 in (285.0 mm)10.04 in (255.0 mm)−1.18 in (−30.0 mm)−10.53%
Circumference88.27 in (2,242.2 mm)88.06 in (2,236.8 mm)−0.21 in (−5.3 mm)−0.24%
Revolutions per mile718719+2+0.24%
True speed with the speedometer calibrated for 285/45R18
IndicatedActual on 285/45R18Actual on 255/40R20Difference
30 mph30.0 mph29.9 mph−0.1 mph
60 mph60.0 mph59.9 mph−0.1 mph
75 mph75.0 mph74.8 mph−0.2 mph

Ride-height change: 255/40R20 lowers the vehicle by 0.03 in (0.9 mm) at the axle (half the −1.7 mm diameter difference).

255/40R20 is 0.07 in shorter than 285/45R18 and 30 mm narrower, a 0.24% difference in overall diameter.

With the speedometer calibrated for 285/45R18, an indicated 60 mph is actually 59.9 mph on 255/40R20, and revolutions per mile change from 718 to 719.

255/40R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 285/45R18.

Size pages

Other comparisons with 285/45R18

Other comparisons with 255/40R20

Source: Calculated from standard tire geometry formulas. Verified . How these numbers are calculated.