245/55R20 vs 255/55R20

255/55R20 is 0.43 in (11.0 mm) taller than 245/55R20, a +1.41% diameter difference. With the speedometer set for 245/55R20, an indicated 60 mph is 60.8 mph on 255/55R20.

Is this a safe swap?

Diameter difference +1.41%: within the ±3% guideline

By overall diameter alone, 255/55R20 falls inside the ±3% range commonly used as a guideline when replacing 245/55R20. 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.

245/55R20 vs 255/55R20 differences
Measurement	245/55R20	255/55R20	Difference	% diff
Overall diameter	30.61 in (777.5 mm)	31.04 in (788.5 mm)	+0.43 in (+11.0 mm)	+1.41%
Sidewall height	5.31 in (134.8 mm)	5.52 in (140.3 mm)	+0.22 in (+5.5 mm)	+4.08%
Section width	9.65 in (245.0 mm)	10.04 in (255.0 mm)	+0.39 in (+10.0 mm)	+4.08%
Circumference	96.16 in (2,442.6 mm)	97.53 in (2,477.1 mm)	+1.36 in (+34.6 mm)	+1.41%
Revolutions per mile	659	650	−9	−1.40%

True speed with the speedometer calibrated for 245/55R20
Indicated	Actual on 245/55R20	Actual on 255/55R20	Difference
30 mph	30.0 mph	30.4 mph	+0.4 mph
60 mph	60.0 mph	60.8 mph	+0.8 mph
75 mph	75.0 mph	76.1 mph	+1.1 mph

Ride-height change: 255/55R20 raises the vehicle by 0.22 in (5.5 mm) at the axle (half the +11.0 mm diameter difference).

255/55R20 is 0.43 in taller than 245/55R20 and 10 mm wider, a 1.41% difference in overall diameter.

With the speedometer calibrated for 245/55R20, an indicated 60 mph is actually 60.8 mph on 255/55R20, and revolutions per mile change from 659 to 650.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 245/55R20

Other comparisons with 255/55R20

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