305/45R20 vs 255/55R20

255/55R20 is 0.24 in (6.0 mm) taller than 305/45R20, a +0.77% diameter difference. With the speedometer set for 305/45R20, an indicated 60 mph is 60.5 mph on 255/55R20.

Is this a safe swap?

Diameter difference +0.77%: under 1%

By overall diameter alone, 255/55R20 is a near match for 305/45R20; 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.

305/45R20 vs 255/55R20 differences
Measurement	305/45R20	255/55R20	Difference	% diff
Overall diameter	30.81 in (782.5 mm)	31.04 in (788.5 mm)	+0.24 in (+6.0 mm)	+0.77%
Sidewall height	5.40 in (137.3 mm)	5.52 in (140.3 mm)	+0.12 in (+3.0 mm)	+2.19%
Section width	12.01 in (305.0 mm)	10.04 in (255.0 mm)	−1.97 in (−50.0 mm)	−16.39%
Circumference	96.78 in (2,458.3 mm)	97.53 in (2,477.1 mm)	+0.74 in (+18.8 mm)	+0.77%
Revolutions per mile	655	650	−5	−0.76%

True speed with the speedometer calibrated for 305/45R20
Indicated	Actual on 305/45R20	Actual on 255/55R20	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.5 mph	+0.5 mph
75 mph	75.0 mph	75.6 mph	+0.6 mph

Ride-height change: 255/55R20 raises the vehicle by 0.12 in (3.0 mm) at the axle (half the +6.0 mm diameter difference).

255/55R20 is 0.24 in taller than 305/45R20 and 50 mm narrower, a 0.77% difference in overall diameter.

With the speedometer calibrated for 305/45R20, an indicated 60 mph is actually 60.5 mph on 255/55R20, and revolutions per mile change from 655 to 650.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 305/45R20

Other comparisons with 255/55R20

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