255/65R18 vs 275/50R20

275/50R20 is 0.22 in (5.7 mm) shorter than 255/65R18, a −0.72% diameter difference. With the speedometer set for 255/65R18, an indicated 60 mph is 59.6 mph on 275/50R20.

Is this a safe swap?

Diameter difference −0.72%: under 1%

By overall diameter alone, 275/50R20 is a near match for 255/65R18; 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.

255/65R18 vs 275/50R20 differences
Measurement	255/65R18	275/50R20	Difference	% diff
Overall diameter	31.05 in (788.7 mm)	30.83 in (783.0 mm)	−0.22 in (−5.7 mm)	−0.72%
Sidewall height	6.53 in (165.8 mm)	5.41 in (137.5 mm)	−1.11 in (−28.2 mm)	−17.04%
Section width	10.04 in (255.0 mm)	10.83 in (275.0 mm)	+0.79 in (+20.0 mm)	+7.84%
Circumference	97.55 in (2,477.8 mm)	96.85 in (2,459.9 mm)	−0.71 in (−17.9 mm)	−0.72%
Revolutions per mile	650	654	+5	+0.73%

True speed with the speedometer calibrated for 255/65R18
Indicated	Actual on 255/65R18	Actual on 275/50R20	Difference
30 mph	30.0 mph	29.8 mph	−0.2 mph
60 mph	60.0 mph	59.6 mph	−0.4 mph
75 mph	75.0 mph	74.5 mph	−0.5 mph

Ride-height change: 275/50R20 lowers the vehicle by 0.11 in (2.9 mm) at the axle (half the −5.7 mm diameter difference).

275/50R20 is 0.22 in shorter than 255/65R18 and 20 mm wider, a 0.72% difference in overall diameter.

With the speedometer calibrated for 255/65R18, an indicated 60 mph is actually 59.6 mph on 275/50R20, and revolutions per mile change from 650 to 654.

275/50R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 255/65R18.

Size pages

Other comparisons with 255/65R18

Other comparisons with 275/50R20

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