TireSizesFinder

275/60R18 vs 255/55R20

255/55R20 is 0.05 in (1.3 mm) taller than 275/60R18, a +0.17% diameter difference. With the speedometer set for 275/60R18, an indicated 60 mph is 60.1 mph on 255/55R20.

Is this a safe swap?

Diameter difference +0.17%: under 1%

By overall diameter alone, 255/55R20 is a near match for 275/60R18; 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: 275/60R18 (30.99 in) vs 255/55R20 (31.04 in)275/60R1830.99 in × 10.83 in255/55R2031.04 in × 10.04 in
Side profile and tread width, drawn to the same scale.
275/60R18 vs 255/55R20 differences
Measurement275/60R18255/55R20Difference% diff
Overall diameter30.99 in (787.2 mm)31.04 in (788.5 mm)+0.05 in (+1.3 mm)+0.17%
Sidewall height6.50 in (165.0 mm)5.52 in (140.3 mm)−0.97 in (−24.7 mm)−15.00%
Section width10.83 in (275.0 mm)10.04 in (255.0 mm)−0.79 in (−20.0 mm)−7.27%
Circumference97.36 in (2,473.1 mm)97.53 in (2,477.1 mm)+0.16 in (+4.1 mm)+0.17%
Revolutions per mile651650−1−0.16%
True speed with the speedometer calibrated for 275/60R18
IndicatedActual on 275/60R18Actual on 255/55R20Difference
30 mph30.0 mph30.0 mph0.0 mph
60 mph60.0 mph60.1 mph+0.1 mph
75 mph75.0 mph75.1 mph+0.1 mph

Ride-height change: 255/55R20 raises the vehicle by 0.03 in (0.6 mm) at the axle (half the +1.3 mm diameter difference).

255/55R20 is 0.05 in taller than 275/60R18 and 20 mm narrower, a 0.17% difference in overall diameter.

With the speedometer calibrated for 275/60R18, an indicated 60 mph is actually 60.1 mph on 255/55R20, and revolutions per mile change from 651 to 650.

255/55R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 275/60R18.

Size pages

Other comparisons with 275/60R18

Other comparisons with 255/55R20

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