235/55R20 vs 255/55R20

255/55R20 is 0.87 in (22.0 mm) taller than 235/55R20, a +2.87% diameter difference. With the speedometer set for 235/55R20, an indicated 60 mph is 61.7 mph on 255/55R20.

Is this a safe swap?

Diameter difference +2.87%: within the ±3% guideline

By overall diameter alone, 255/55R20 falls inside the ±3% range commonly used as a guideline when replacing 235/55R20. 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.

235/55R20 vs 255/55R20 differences
Measurement	235/55R20	255/55R20	Difference	% diff
Overall diameter	30.18 in (766.5 mm)	31.04 in (788.5 mm)	+0.87 in (+22.0 mm)	+2.87%
Sidewall height	5.09 in (129.3 mm)	5.52 in (140.3 mm)	+0.43 in (+11.0 mm)	+8.51%
Section width	9.25 in (235.0 mm)	10.04 in (255.0 mm)	+0.79 in (+20.0 mm)	+8.51%
Circumference	94.80 in (2,408.0 mm)	97.53 in (2,477.1 mm)	+2.72 in (+69.1 mm)	+2.87%
Revolutions per mile	668	650	−19	−2.79%

True speed with the speedometer calibrated for 235/55R20
Indicated	Actual on 235/55R20	Actual on 255/55R20	Difference
30 mph	30.0 mph	30.9 mph	+0.9 mph
60 mph	60.0 mph	61.7 mph	+1.7 mph
75 mph	75.0 mph	77.2 mph	+2.2 mph

Ride-height change: 255/55R20 raises the vehicle by 0.43 in (11.0 mm) at the axle (half the +22.0 mm diameter difference).

255/55R20 is 0.87 in taller than 235/55R20 and 20 mm wider, a 2.87% difference in overall diameter.

With the speedometer calibrated for 235/55R20, an indicated 60 mph is actually 61.7 mph on 255/55R20, and revolutions per mile change from 668 to 650.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 235/55R20

Other comparisons with 255/55R20

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