205/50R16 vs 205/40R18

205/40R18 is 0.39 in (9.8 mm) taller than 205/50R16, a +1.60% diameter difference. With the speedometer set for 205/50R16, an indicated 60 mph is 61.0 mph on 205/40R18.

Is this a safe swap?

Diameter difference +1.60%: within the ±3% guideline

By overall diameter alone, 205/40R18 falls inside the ±3% range commonly used as a guideline when replacing 205/50R16. 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.

205/50R16 vs 205/40R18 differences
Measurement	205/50R16	205/40R18	Difference	% diff
Overall diameter	24.07 in (611.4 mm)	24.46 in (621.2 mm)	+0.39 in (+9.8 mm)	+1.60%
Sidewall height	4.04 in (102.5 mm)	3.23 in (82.0 mm)	−0.81 in (−20.5 mm)	−20.00%
Section width	8.07 in (205.0 mm)	8.07 in (205.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	75.62 in (1,920.8 mm)	76.83 in (1,951.6 mm)	+1.21 in (+30.8 mm)	+1.60%
Revolutions per mile	838	825	−13	−1.58%

True speed with the speedometer calibrated for 205/50R16
Indicated	Actual on 205/50R16	Actual on 205/40R18	Difference
30 mph	30.0 mph	30.5 mph	+0.5 mph
60 mph	60.0 mph	61.0 mph	+1.0 mph
75 mph	75.0 mph	76.2 mph	+1.2 mph

Ride-height change: 205/40R18 raises the vehicle by 0.19 in (4.9 mm) at the axle (half the +9.8 mm diameter difference).

205/40R18 is 0.39 in taller than 205/50R16 and the same width, a 1.60% difference in overall diameter.

With the speedometer calibrated for 205/50R16, an indicated 60 mph is actually 61.0 mph on 205/40R18, and revolutions per mile change from 838 to 825.

205/40R18 mounts on a 18-inch wheel, a plus-2 fitment from the 16-inch wheel of 205/50R16.

Size pages

Other comparisons with 205/50R16

Other comparisons with 205/40R18

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