275/40R18 vs 275/30R20

275/30R20 is 0.17 in (4.2 mm) shorter than 275/40R18, a −0.62% diameter difference. With the speedometer set for 275/40R18, an indicated 60 mph is 59.6 mph on 275/30R20.

Is this a safe swap?

Diameter difference −0.62%: under 1%

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

275/40R18 vs 275/30R20 differences
Measurement	275/40R18	275/30R20	Difference	% diff
Overall diameter	26.66 in (677.2 mm)	26.50 in (673.0 mm)	−0.17 in (−4.2 mm)	−0.62%
Sidewall height	4.33 in (110.0 mm)	3.25 in (82.5 mm)	−1.08 in (−27.5 mm)	−25.00%
Section width	10.83 in (275.0 mm)	10.83 in (275.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	83.76 in (2,127.5 mm)	83.24 in (2,114.3 mm)	−0.52 in (−13.2 mm)	−0.62%
Revolutions per mile	756	761	+5	+0.62%

True speed with the speedometer calibrated for 275/40R18
Indicated	Actual on 275/40R18	Actual on 275/30R20	Difference
30 mph	30.0 mph	29.8 mph	−0.2 mph
60 mph	60.0 mph	59.6 mph	−0.4 mph
75 mph	75.0 mph	74.5 mph	−0.5 mph

Ride-height change: 275/30R20 lowers the vehicle by 0.08 in (2.1 mm) at the axle (half the −4.2 mm diameter difference).

275/30R20 is 0.17 in shorter than 275/40R18 and the same width, a 0.62% difference in overall diameter.

With the speedometer calibrated for 275/40R18, an indicated 60 mph is actually 59.6 mph on 275/30R20, and revolutions per mile change from 756 to 761.

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

Size pages

Other comparisons with 275/40R18

Other comparisons with 275/30R20

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