275/70R18 vs 305/55R20

305/55R20 is 0.05 in (1.3 mm) taller than 275/70R18, a +0.15% diameter difference. With the speedometer set for 275/70R18, an indicated 60 mph is 60.1 mph on 305/55R20.

Is this a safe swap?

Diameter difference +0.15%: under 1%

By overall diameter alone, 305/55R20 is a near match for 275/70R18; 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.

275/70R18 vs 305/55R20 differences
Measurement	275/70R18	305/55R20	Difference	% diff
Overall diameter	33.16 in (842.2 mm)	33.21 in (843.5 mm)	+0.05 in (+1.3 mm)	+0.15%
Sidewall height	7.58 in (192.5 mm)	6.60 in (167.8 mm)	−0.97 in (−24.7 mm)	−12.86%
Section width	10.83 in (275.0 mm)	12.01 in (305.0 mm)	+1.18 in (+30.0 mm)	+10.91%
Circumference	104.17 in (2,645.8 mm)	104.33 in (2,649.9 mm)	+0.16 in (+4.1 mm)	+0.15%
Revolutions per mile	608	607	−1	−0.15%

True speed with the speedometer calibrated for 275/70R18
Indicated	Actual on 275/70R18	Actual on 305/55R20	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: 305/55R20 raises the vehicle by 0.03 in (0.6 mm) at the axle (half the +1.3 mm diameter difference).

305/55R20 is 0.05 in taller than 275/70R18 and 30 mm wider, a 0.15% difference in overall diameter.

With the speedometer calibrated for 275/70R18, an indicated 60 mph is actually 60.1 mph on 305/55R20, and revolutions per mile change from 608 to 607.

305/55R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 275/70R18.

Size pages

Other comparisons with 275/70R18

Other comparisons with 305/55R20

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