185/60R16 vs 215/40R18

215/40R18 is 0.03 in (0.8 mm) taller than 185/60R16, a +0.13% diameter difference. With the speedometer set for 185/60R16, an indicated 60 mph is 60.1 mph on 215/40R18.

Is this a safe swap?

Diameter difference +0.13%: under 1%

By overall diameter alone, 215/40R18 is a near match for 185/60R16; 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.

185/60R16 vs 215/40R18 differences
Measurement	185/60R16	215/40R18	Difference	% diff
Overall diameter	24.74 in (628.4 mm)	24.77 in (629.2 mm)	+0.03 in (+0.8 mm)	+0.13%
Sidewall height	4.37 in (111.0 mm)	3.39 in (86.0 mm)	−0.98 in (−25.0 mm)	−22.52%
Section width	7.28 in (185.0 mm)	8.46 in (215.0 mm)	+1.18 in (+30.0 mm)	+16.22%
Circumference	77.72 in (1,974.2 mm)	77.82 in (1,976.7 mm)	+0.10 in (+2.5 mm)	+0.13%
Revolutions per mile	815	814	−1	−0.13%

True speed with the speedometer calibrated for 185/60R16
Indicated	Actual on 185/60R16	Actual on 215/40R18	Difference
30 mph	30.0 mph	30.0 mph	0.0 mph
60 mph	60.0 mph	60.1 mph	+0.1 mph
75 mph	75.0 mph	75.1 mph	+0.1 mph

Ride-height change: 215/40R18 raises the vehicle by 0.02 in (0.4 mm) at the axle (half the +0.8 mm diameter difference).

215/40R18 is 0.03 in taller than 185/60R16 and 30 mm wider, a 0.13% difference in overall diameter.

With the speedometer calibrated for 185/60R16, an indicated 60 mph is actually 60.1 mph on 215/40R18, and revolutions per mile change from 815 to 814.

215/40R18 mounts on a 18-inch wheel, a plus-2 fitment from the 16-inch wheel of 185/60R16.

Size pages

Other comparisons with 185/60R16

Other comparisons with 215/40R18

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