215/55R16 vs 225/40R18

225/40R18 is 0.22 in (5.7 mm) shorter than 215/55R16, a −0.89% diameter difference. With the speedometer set for 215/55R16, an indicated 60 mph is 59.5 mph on 225/40R18.

Is this a safe swap?

Diameter difference −0.89%: under 1%

By overall diameter alone, 225/40R18 is a near match for 215/55R16; 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.

215/55R16 vs 225/40R18 differences
Measurement	215/55R16	225/40R18	Difference	% diff
Overall diameter	25.31 in (642.9 mm)	25.09 in (637.2 mm)	−0.22 in (−5.7 mm)	−0.89%
Sidewall height	4.66 in (118.3 mm)	3.54 in (90.0 mm)	−1.11 in (−28.2 mm)	−23.89%
Section width	8.46 in (215.0 mm)	8.86 in (225.0 mm)	+0.39 in (+10.0 mm)	+4.65%
Circumference	79.52 in (2,019.7 mm)	78.81 in (2,001.8 mm)	−0.71 in (−17.9 mm)	−0.89%
Revolutions per mile	797	804	+7	+0.89%

True speed with the speedometer calibrated for 215/55R16
Indicated	Actual on 215/55R16	Actual on 225/40R18	Difference
30 mph	30.0 mph	29.7 mph	−0.3 mph
60 mph	60.0 mph	59.5 mph	−0.5 mph
75 mph	75.0 mph	74.3 mph	−0.7 mph

Ride-height change: 225/40R18 lowers the vehicle by 0.11 in (2.8 mm) at the axle (half the −5.7 mm diameter difference).

225/40R18 is 0.22 in shorter than 215/55R16 and 10 mm wider, a 0.89% difference in overall diameter.

With the speedometer calibrated for 215/55R16, an indicated 60 mph is actually 59.5 mph on 225/40R18, and revolutions per mile change from 797 to 804.

225/40R18 mounts on a 18-inch wheel, a plus-2 fitment from the 16-inch wheel of 215/55R16.

Size pages

Other comparisons with 215/55R16

Other comparisons with 225/40R18

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