235/45R18 vs 275/30R20

275/30R20 is 0.17 in (4.3 mm) taller than 235/45R18, a +0.64% diameter difference. With the speedometer set for 235/45R18, an indicated 60 mph is 60.4 mph on 275/30R20.

Is this a safe swap?

Diameter difference +0.64%: under 1%

By overall diameter alone, 275/30R20 is a near match for 235/45R18; 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.

235/45R18 vs 275/30R20 differences
Measurement	235/45R18	275/30R20	Difference	% diff
Overall diameter	26.33 in (668.7 mm)	26.50 in (673.0 mm)	+0.17 in (+4.3 mm)	+0.64%
Sidewall height	4.16 in (105.8 mm)	3.25 in (82.5 mm)	−0.92 in (−23.2 mm)	−21.99%
Section width	9.25 in (235.0 mm)	10.83 in (275.0 mm)	+1.57 in (+40.0 mm)	+17.02%
Circumference	82.71 in (2,100.8 mm)	83.24 in (2,114.3 mm)	+0.53 in (+13.5 mm)	+0.64%
Revolutions per mile	766	761	−5	−0.64%

True speed with the speedometer calibrated for 235/45R18
Indicated	Actual on 235/45R18	Actual on 275/30R20	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.4 mph	+0.4 mph
75 mph	75.0 mph	75.5 mph	+0.5 mph

Ride-height change: 275/30R20 raises the vehicle by 0.08 in (2.1 mm) at the axle (half the +4.3 mm diameter difference).

275/30R20 is 0.17 in taller than 235/45R18 and 40 mm wider, a 0.64% difference in overall diameter.

With the speedometer calibrated for 235/45R18, an indicated 60 mph is actually 60.4 mph on 275/30R20, and revolutions per mile change from 766 to 761.

275/30R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 235/45R18.

Size pages

Other comparisons with 235/45R18

Other comparisons with 275/30R20

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