305/50R20 vs 255/60R20

255/60R20 is 0.04 in (1.0 mm) taller than 305/50R20, a +0.12% diameter difference. With the speedometer set for 305/50R20, an indicated 60 mph is 60.1 mph on 255/60R20.

Is this a safe swap?

Diameter difference +0.12%: under 1%

By overall diameter alone, 255/60R20 is a near match for 305/50R20; 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.

305/50R20 vs 255/60R20 differences
Measurement	305/50R20	255/60R20	Difference	% diff
Overall diameter	32.01 in (813.0 mm)	32.05 in (814.0 mm)	+0.04 in (+1.0 mm)	+0.12%
Sidewall height	6.00 in (152.5 mm)	6.02 in (153.0 mm)	+0.02 in (+0.5 mm)	+0.33%
Section width	12.01 in (305.0 mm)	10.04 in (255.0 mm)	−1.97 in (−50.0 mm)	−16.39%
Circumference	100.56 in (2,554.1 mm)	100.68 in (2,557.3 mm)	+0.12 in (+3.1 mm)	+0.12%
Revolutions per mile	630	629	−1	−0.12%

True speed with the speedometer calibrated for 305/50R20
Indicated	Actual on 305/50R20	Actual on 255/60R20	Difference
30 mph	30.0 mph	30.0 mph	0.0 mph
60 mph	60.0 mph	60.1 mph	+0.1 mph
75 mph	75.0 mph	75.1 mph	+0.1 mph

Ride-height change: 255/60R20 raises the vehicle by 0.02 in (0.5 mm) at the axle (half the +1.0 mm diameter difference).

255/60R20 is 0.04 in taller than 305/50R20 and 50 mm narrower, a 0.12% difference in overall diameter.

With the speedometer calibrated for 305/50R20, an indicated 60 mph is actually 60.1 mph on 255/60R20, and revolutions per mile change from 630 to 629.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 305/50R20

Other comparisons with 255/60R20

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