205/55R16 vs 225/40R18

225/40R18 is 0.21 in (5.3 mm) taller than 205/55R16, a +0.84% diameter difference. With the speedometer set for 205/55R16, an indicated 60 mph is 60.5 mph on 225/40R18.

Is this a safe swap?

Diameter difference +0.84%: under 1%

By overall diameter alone, 225/40R18 is a near match for 205/55R16; 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.

205/55R16 vs 225/40R18 differences
Measurement	205/55R16	225/40R18	Difference	% diff
Overall diameter	24.88 in (631.9 mm)	25.09 in (637.2 mm)	+0.21 in (+5.3 mm)	+0.84%
Sidewall height	4.44 in (112.8 mm)	3.54 in (90.0 mm)	−0.90 in (−22.7 mm)	−20.18%
Section width	8.07 in (205.0 mm)	8.86 in (225.0 mm)	+0.79 in (+20.0 mm)	+9.76%
Circumference	78.16 in (1,985.2 mm)	78.81 in (2,001.8 mm)	+0.66 in (+16.7 mm)	+0.84%
Revolutions per mile	811	804	−7	−0.83%

True speed with the speedometer calibrated for 205/55R16
Indicated	Actual on 205/55R16	Actual on 225/40R18	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	75.6 mph	+0.6 mph

Ride-height change: 225/40R18 raises the vehicle by 0.10 in (2.7 mm) at the axle (half the +5.3 mm diameter difference).

225/40R18 is 0.21 in taller than 205/55R16 and 20 mm wider, a 0.84% difference in overall diameter.

With the speedometer calibrated for 205/55R16, an indicated 60 mph is actually 60.5 mph on 225/40R18, and revolutions per mile change from 811 to 804.

225/40R18 mounts on a 18-inch wheel, a plus-2 fitment from the 16-inch wheel of 205/55R16.

Size pages

Other comparisons with 205/55R16

Other comparisons with 225/40R18

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