225/50R16 vs 225/40R18

225/40R18 is 0.23 in (5.8 mm) taller than 225/50R16, a +0.92% diameter difference. With the speedometer set for 225/50R16, an indicated 60 mph is 60.6 mph on 225/40R18.

Is this a safe swap?

Diameter difference +0.92%: under 1%

By overall diameter alone, 225/40R18 is a near match for 225/50R16; 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.

225/50R16 vs 225/40R18 differences
Measurement	225/50R16	225/40R18	Difference	% diff
Overall diameter	24.86 in (631.4 mm)	25.09 in (637.2 mm)	+0.23 in (+5.8 mm)	+0.92%
Sidewall height	4.43 in (112.5 mm)	3.54 in (90.0 mm)	−0.89 in (−22.5 mm)	−20.00%
Section width	8.86 in (225.0 mm)	8.86 in (225.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	78.09 in (1,983.6 mm)	78.81 in (2,001.8 mm)	+0.72 in (+18.2 mm)	+0.92%
Revolutions per mile	811	804	−7	−0.91%

True speed with the speedometer calibrated for 225/50R16
Indicated	Actual on 225/50R16	Actual on 225/40R18	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.6 mph	+0.6 mph
75 mph	75.0 mph	75.7 mph	+0.7 mph

Ride-height change: 225/40R18 raises the vehicle by 0.11 in (2.9 mm) at the axle (half the +5.8 mm diameter difference).

225/40R18 is 0.23 in taller than 225/50R16 and the same width, a 0.92% difference in overall diameter.

With the speedometer calibrated for 225/50R16, an indicated 60 mph is actually 60.6 mph on 225/40R18, and revolutions per mile change from 811 to 804.

225/40R18 mounts on a 18-inch wheel, a plus-2 fitment from the 16-inch wheel of 225/50R16.

Size pages

Other comparisons with 225/50R16

Other comparisons with 225/40R18

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