225/40R18 vs 225/30R20

225/30R20 is 0.23 in (5.8 mm) taller than 225/40R18, a +0.91% diameter difference. With the speedometer set for 225/40R18, an indicated 60 mph is 60.5 mph on 225/30R20.

Is this a safe swap?

Diameter difference +0.91%: under 1%

By overall diameter alone, 225/30R20 is a near match for 225/40R18; 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.

225/40R18 vs 225/30R20 differences
Measurement	225/40R18	225/30R20	Difference	% diff
Overall diameter	25.09 in (637.2 mm)	25.31 in (643.0 mm)	+0.23 in (+5.8 mm)	+0.91%
Sidewall height	3.54 in (90.0 mm)	2.66 in (67.5 mm)	−0.89 in (−22.5 mm)	−25.00%
Section width	8.86 in (225.0 mm)	8.86 in (225.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	78.81 in (2,001.8 mm)	79.53 in (2,020.0 mm)	+0.72 in (+18.2 mm)	+0.91%
Revolutions per mile	804	797	−7	−0.90%

True speed with the speedometer calibrated for 225/40R18
Indicated	Actual on 225/40R18	Actual on 225/30R20	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.5 mph	+0.5 mph
75 mph	75.0 mph	75.7 mph	+0.7 mph

Ride-height change: 225/30R20 raises the vehicle by 0.11 in (2.9 mm) at the axle (half the +5.8 mm diameter difference).

225/30R20 is 0.23 in taller than 225/40R18 and the same width, a 0.91% difference in overall diameter.

With the speedometer calibrated for 225/40R18, an indicated 60 mph is actually 60.5 mph on 225/30R20, and revolutions per mile change from 804 to 797.

225/30R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 225/40R18.

Size pages

Other comparisons with 225/40R18

Other comparisons with 225/30R20

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