195/55R16 vs 205/40R18

205/40R18 is 0.01 in (0.3 mm) taller than 195/55R16, a +0.05% diameter difference. With the speedometer set for 195/55R16, an indicated 60 mph is 60.0 mph on 205/40R18.

Is this a safe swap?

Diameter difference +0.05%: under 1%

By overall diameter alone, 205/40R18 is a near match for 195/55R16; 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.

195/55R16 vs 205/40R18 differences
Measurement	195/55R16	205/40R18	Difference	% diff
Overall diameter	24.44 in (620.9 mm)	24.46 in (621.2 mm)	+0.01 in (+0.3 mm)	+0.05%
Sidewall height	4.22 in (107.3 mm)	3.23 in (82.0 mm)	−0.99 in (−25.2 mm)	−23.54%
Section width	7.68 in (195.0 mm)	8.07 in (205.0 mm)	+0.39 in (+10.0 mm)	+5.13%
Circumference	76.80 in (1,950.6 mm)	76.83 in (1,951.6 mm)	+0.04 in (+0.9 mm)	+0.05%
Revolutions per mile	825	825	0	−0.05%

True speed with the speedometer calibrated for 195/55R16
Indicated	Actual on 195/55R16	Actual on 205/40R18
30 mph	30.0 mph	30.0 mph
60 mph	60.0 mph	60.0 mph
75 mph	75.0 mph	75.0 mph

Ride-height change: 205/40R18 raises the vehicle by 0.01 in (0.2 mm) at the axle (half the +0.3 mm diameter difference).

205/40R18 is 0.01 in taller than 195/55R16 and 10 mm wider, a 0.05% difference in overall diameter.

With the speedometer calibrated for 195/55R16, an indicated 60 mph is actually 60.0 mph on 205/40R18, and revolutions per mile change from 825 to 825.

205/40R18 mounts on a 18-inch wheel, a plus-2 fitment from the 16-inch wheel of 195/55R16.

Size pages

Other comparisons with 195/55R16

Other comparisons with 205/40R18

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