275/55R18 vs 255/50R20

255/50R20 is 0.13 in (3.3 mm) taller than 275/55R18, a +0.43% diameter difference. With the speedometer set for 275/55R18, an indicated 60 mph is 60.3 mph on 255/50R20.

Is this a safe swap?

Diameter difference +0.43%: under 1%

By overall diameter alone, 255/50R20 is a near match for 275/55R18; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

Side profile and tread width, drawn to the same scale.

275/55R18 vs 255/50R20 differences
Measurement	275/55R18	255/50R20	Difference	% diff
Overall diameter	29.91 in (759.7 mm)	30.04 in (763.0 mm)	+0.13 in (+3.3 mm)	+0.43%
Sidewall height	5.95 in (151.3 mm)	5.02 in (127.5 mm)	−0.94 in (−23.7 mm)	−15.70%
Section width	10.83 in (275.0 mm)	10.04 in (255.0 mm)	−0.79 in (−20.0 mm)	−7.27%
Circumference	93.96 in (2,386.7 mm)	94.37 in (2,397.0 mm)	+0.41 in (+10.4 mm)	+0.43%
Revolutions per mile	674	671	−3	−0.43%

True speed with the speedometer calibrated for 275/55R18
Indicated	Actual on 275/55R18	Actual on 255/50R20	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.3 mph	+0.3 mph
75 mph	75.0 mph	75.3 mph	+0.3 mph

Ride-height change: 255/50R20 raises the vehicle by 0.06 in (1.6 mm) at the axle (half the +3.3 mm diameter difference).

255/50R20 is 0.13 in taller than 275/55R18 and 20 mm narrower, a 0.43% difference in overall diameter.

With the speedometer calibrated for 275/55R18, an indicated 60 mph is actually 60.3 mph on 255/50R20, and revolutions per mile change from 674 to 671.

255/50R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 275/55R18.

Size pages

Other comparisons with 275/55R18

Other comparisons with 255/50R20

Source: Calculated from standard tire geometry formulas. Verified 2026-09-27. How these numbers are calculated.