185/60R16 vs 205/40R18

205/40R18 is 0.28 in (7.2 mm) shorter than 185/60R16, a −1.15% diameter difference. With the speedometer set for 185/60R16, an indicated 60 mph is 59.3 mph on 205/40R18.

Is this a safe swap?

Diameter difference −1.15%: within the ±3% guideline

By overall diameter alone, 205/40R18 falls inside the ±3% range commonly used as a guideline when replacing 185/60R16. 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 205/40R18 differences
Measurement	185/60R16	205/40R18	Difference	% diff
Overall diameter	24.74 in (628.4 mm)	24.46 in (621.2 mm)	−0.28 in (−7.2 mm)	−1.15%
Sidewall height	4.37 in (111.0 mm)	3.23 in (82.0 mm)	−1.14 in (−29.0 mm)	−26.13%
Section width	7.28 in (185.0 mm)	8.07 in (205.0 mm)	+0.79 in (+20.0 mm)	+10.81%
Circumference	77.72 in (1,974.2 mm)	76.83 in (1,951.6 mm)	−0.89 in (−22.6 mm)	−1.15%
Revolutions per mile	815	825	+9	+1.16%

True speed with the speedometer calibrated for 185/60R16
Indicated	Actual on 185/60R16	Actual on 205/40R18	Difference
30 mph	30.0 mph	29.7 mph	−0.3 mph
60 mph	60.0 mph	59.3 mph	−0.7 mph
75 mph	75.0 mph	74.1 mph	−0.9 mph

Ride-height change: 205/40R18 lowers the vehicle by 0.14 in (3.6 mm) at the axle (half the −7.2 mm diameter difference).

205/40R18 is 0.28 in shorter than 185/60R16 and 20 mm wider, a 1.15% difference in overall diameter.

With the speedometer calibrated for 185/60R16, an indicated 60 mph is actually 59.3 mph on 205/40R18, and revolutions per mile change from 815 to 825.

205/40R18 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 205/40R18

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