TireSizesFinder

235/55R18 vs 255/40R20

255/40R20 is 0.15 in (3.7 mm) shorter than 235/55R18, a −0.52% diameter difference. With the speedometer set for 235/55R18, an indicated 60 mph is 59.7 mph on 255/40R20.

Is this a safe swap?

Diameter difference −0.52%: under 1%

By overall diameter alone, 255/40R20 is a near match for 235/55R18; 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: 235/55R18 (28.18 in) vs 255/40R20 (28.03 in)235/55R1828.18 in × 9.25 in255/40R2028.03 in × 10.04 in
Side profile and tread width, drawn to the same scale.
235/55R18 vs 255/40R20 differences
Measurement235/55R18255/40R20Difference% diff
Overall diameter28.18 in (715.7 mm)28.03 in (712.0 mm)−0.15 in (−3.7 mm)−0.52%
Sidewall height5.09 in (129.3 mm)4.02 in (102.0 mm)−1.07 in (−27.2 mm)−21.08%
Section width9.25 in (235.0 mm)10.04 in (255.0 mm)+0.79 in (+20.0 mm)+8.51%
Circumference88.52 in (2,248.4 mm)88.06 in (2,236.8 mm)−0.46 in (−11.6 mm)−0.52%
Revolutions per mile716719+4+0.52%
True speed with the speedometer calibrated for 235/55R18
IndicatedActual on 235/55R18Actual on 255/40R20Difference
30 mph30.0 mph29.8 mph−0.2 mph
60 mph60.0 mph59.7 mph−0.3 mph
75 mph75.0 mph74.6 mph−0.4 mph

Ride-height change: 255/40R20 lowers the vehicle by 0.07 in (1.9 mm) at the axle (half the −3.7 mm diameter difference).

255/40R20 is 0.15 in shorter than 235/55R18 and 20 mm wider, a 0.52% difference in overall diameter.

With the speedometer calibrated for 235/55R18, an indicated 60 mph is actually 59.7 mph on 255/40R20, and revolutions per mile change from 716 to 719.

255/40R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 235/55R18.

Size pages

Other comparisons with 235/55R18

Other comparisons with 255/40R20

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