215/50R18 vs 275/30R20

275/30R20 is 0.03 in (0.8 mm) taller than 215/50R18, a +0.12% diameter difference. With the speedometer set for 215/50R18, an indicated 60 mph is 60.1 mph on 275/30R20.

Is this a safe swap?

Diameter difference +0.12%: under 1%

By overall diameter alone, 275/30R20 is a near match for 215/50R18; 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.

215/50R18 vs 275/30R20 differences
Measurement	215/50R18	275/30R20	Difference	% diff
Overall diameter	26.46 in (672.2 mm)	26.50 in (673.0 mm)	+0.03 in (+0.8 mm)	+0.12%
Sidewall height	4.23 in (107.5 mm)	3.25 in (82.5 mm)	−0.98 in (−25.0 mm)	−23.26%
Section width	8.46 in (215.0 mm)	10.83 in (275.0 mm)	+2.36 in (+60.0 mm)	+27.91%
Circumference	83.14 in (2,111.8 mm)	83.24 in (2,114.3 mm)	+0.10 in (+2.5 mm)	+0.12%
Revolutions per mile	762	761	−1	−0.12%

True speed with the speedometer calibrated for 215/50R18
Indicated	Actual on 215/50R18	Actual on 275/30R20	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: 275/30R20 raises the vehicle by 0.02 in (0.4 mm) at the axle (half the +0.8 mm diameter difference).

275/30R20 is 0.03 in taller than 215/50R18 and 60 mm wider, a 0.12% difference in overall diameter.

With the speedometer calibrated for 215/50R18, an indicated 60 mph is actually 60.1 mph on 275/30R20, and revolutions per mile change from 762 to 761.

275/30R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 215/50R18.

Size pages

Other comparisons with 215/50R18

Other comparisons with 275/30R20

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