205/55R16 vs 215/40R18

215/40R18 is 0.11 in (2.7 mm) shorter than 205/55R16, a −0.43% diameter difference. With the speedometer set for 205/55R16, an indicated 60 mph is 59.7 mph on 215/40R18.

Is this a safe swap?

Diameter difference −0.43%: under 1%

By overall diameter alone, 215/40R18 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 215/40R18 differences
Measurement	205/55R16	215/40R18	Difference	% diff
Overall diameter	24.88 in (631.9 mm)	24.77 in (629.2 mm)	−0.11 in (−2.7 mm)	−0.43%
Sidewall height	4.44 in (112.8 mm)	3.39 in (86.0 mm)	−1.05 in (−26.7 mm)	−23.73%
Section width	8.07 in (205.0 mm)	8.46 in (215.0 mm)	+0.39 in (+10.0 mm)	+4.88%
Circumference	78.16 in (1,985.2 mm)	77.82 in (1,976.7 mm)	−0.33 in (−8.5 mm)	−0.43%
Revolutions per mile	811	814	+3	+0.43%

True speed with the speedometer calibrated for 205/55R16
Indicated	Actual on 205/55R16	Actual on 215/40R18	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.7 mph	−0.3 mph
75 mph	75.0 mph	74.7 mph	−0.3 mph

Ride-height change: 215/40R18 lowers the vehicle by 0.05 in (1.3 mm) at the axle (half the −2.7 mm diameter difference).

215/40R18 is 0.11 in shorter than 205/55R16 and 10 mm wider, a 0.43% difference in overall diameter.

With the speedometer calibrated for 205/55R16, an indicated 60 mph is actually 59.7 mph on 215/40R18, and revolutions per mile change from 811 to 814.

215/40R18 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 215/40R18

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