255/30R20 vs 275/30R20

275/30R20 is 0.47 in (12.0 mm) taller than 255/30R20, a +1.82% diameter difference. With the speedometer set for 255/30R20, an indicated 60 mph is 61.1 mph on 275/30R20.

Is this a safe swap?

Diameter difference +1.82%: within the ±3% guideline

By overall diameter alone, 275/30R20 falls inside the ±3% range commonly used as a guideline when replacing 255/30R20. 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.

255/30R20 vs 275/30R20 differences
Measurement	255/30R20	275/30R20	Difference	% diff
Overall diameter	26.02 in (661.0 mm)	26.50 in (673.0 mm)	+0.47 in (+12.0 mm)	+1.82%
Sidewall height	3.01 in (76.5 mm)	3.25 in (82.5 mm)	+0.24 in (+6.0 mm)	+7.84%
Section width	10.04 in (255.0 mm)	10.83 in (275.0 mm)	+0.79 in (+20.0 mm)	+7.84%
Circumference	81.76 in (2,076.6 mm)	83.24 in (2,114.3 mm)	+1.48 in (+37.7 mm)	+1.82%
Revolutions per mile	775	761	−14	−1.78%

True speed with the speedometer calibrated for 255/30R20
Indicated	Actual on 255/30R20	Actual on 275/30R20	Difference
30 mph	30.0 mph	30.5 mph	+0.5 mph
60 mph	60.0 mph	61.1 mph	+1.1 mph
75 mph	75.0 mph	76.4 mph	+1.4 mph

Ride-height change: 275/30R20 raises the vehicle by 0.24 in (6.0 mm) at the axle (half the +12.0 mm diameter difference).

275/30R20 is 0.47 in taller than 255/30R20 and 20 mm wider, a 1.82% difference in overall diameter.

With the speedometer calibrated for 255/30R20, an indicated 60 mph is actually 61.1 mph on 275/30R20, and revolutions per mile change from 775 to 761.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 255/30R20

Other comparisons with 275/30R20

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