255/70R18 vs 275/55R20

275/55R20 is 0.15 in (3.7 mm) shorter than 255/70R18, a −0.45% diameter difference. With the speedometer set for 255/70R18, an indicated 60 mph is 59.7 mph on 275/55R20.

Is this a safe swap?

Diameter difference −0.45%: under 1%

By overall diameter alone, 275/55R20 is a near match for 255/70R18; 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.

255/70R18 vs 275/55R20 differences
Measurement	255/70R18	275/55R20	Difference	% diff
Overall diameter	32.06 in (814.2 mm)	31.91 in (810.5 mm)	−0.15 in (−3.7 mm)	−0.45%
Sidewall height	7.03 in (178.5 mm)	5.95 in (151.3 mm)	−1.07 in (−27.2 mm)	−15.27%
Section width	10.04 in (255.0 mm)	10.83 in (275.0 mm)	+0.79 in (+20.0 mm)	+7.84%
Circumference	100.70 in (2,557.9 mm)	100.25 in (2,546.3 mm)	−0.46 in (−11.6 mm)	−0.45%
Revolutions per mile	629	632	+3	+0.46%

True speed with the speedometer calibrated for 255/70R18
Indicated	Actual on 255/70R18	Actual on 275/55R20	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.7 mph	−0.3 mph
75 mph	75.0 mph	74.7 mph	−0.3 mph

Ride-height change: 275/55R20 lowers the vehicle by 0.07 in (1.9 mm) at the axle (half the −3.7 mm diameter difference).

275/55R20 is 0.15 in shorter than 255/70R18 and 20 mm wider, a 0.45% difference in overall diameter.

With the speedometer calibrated for 255/70R18, an indicated 60 mph is actually 59.7 mph on 275/55R20, and revolutions per mile change from 629 to 632.

275/55R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 255/70R18.

Size pages

Other comparisons with 255/70R18

Other comparisons with 275/55R20

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