265/40R18 vs 275/30R20

275/30R20 is 0.15 in (3.8 mm) taller than 265/40R18, a +0.57% diameter difference. With the speedometer set for 265/40R18, an indicated 60 mph is 60.3 mph on 275/30R20.

Is this a safe swap?

Diameter difference +0.57%: under 1%

By overall diameter alone, 275/30R20 is a near match for 265/40R18; 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.

265/40R18 vs 275/30R20 differences
Measurement	265/40R18	275/30R20	Difference	% diff
Overall diameter	26.35 in (669.2 mm)	26.50 in (673.0 mm)	+0.15 in (+3.8 mm)	+0.57%
Sidewall height	4.17 in (106.0 mm)	3.25 in (82.5 mm)	−0.93 in (−23.5 mm)	−22.17%
Section width	10.43 in (265.0 mm)	10.83 in (275.0 mm)	+0.39 in (+10.0 mm)	+3.77%
Circumference	82.77 in (2,102.4 mm)	83.24 in (2,114.3 mm)	+0.47 in (+11.9 mm)	+0.57%
Revolutions per mile	765	761	−4	−0.56%

True speed with the speedometer calibrated for 265/40R18
Indicated	Actual on 265/40R18	Actual on 275/30R20	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.3 mph	+0.3 mph
75 mph	75.0 mph	75.4 mph	+0.4 mph

Ride-height change: 275/30R20 raises the vehicle by 0.07 in (1.9 mm) at the axle (half the +3.8 mm diameter difference).

275/30R20 is 0.15 in taller than 265/40R18 and 10 mm wider, a 0.57% difference in overall diameter.

With the speedometer calibrated for 265/40R18, an indicated 60 mph is actually 60.3 mph on 275/30R20, and revolutions per mile change from 765 to 761.

275/30R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 265/40R18.

Size pages

Other comparisons with 265/40R18

Other comparisons with 275/30R20

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