215/55R16 vs 235/40R18

235/40R18 is 0.09 in (2.3 mm) taller than 215/55R16, a +0.36% diameter difference. With the speedometer set for 215/55R16, an indicated 60 mph is 60.2 mph on 235/40R18.

Is this a safe swap?

Diameter difference +0.36%: under 1%

By overall diameter alone, 235/40R18 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 235/40R18 differences
Measurement	215/55R16	235/40R18	Difference	% diff
Overall diameter	25.31 in (642.9 mm)	25.40 in (645.2 mm)	+0.09 in (+2.3 mm)	+0.36%
Sidewall height	4.66 in (118.3 mm)	3.70 in (94.0 mm)	−0.95 in (−24.2 mm)	−20.51%
Section width	8.46 in (215.0 mm)	9.25 in (235.0 mm)	+0.79 in (+20.0 mm)	+9.30%
Circumference	79.52 in (2,019.7 mm)	79.80 in (2,027.0 mm)	+0.28 in (+7.2 mm)	+0.36%
Revolutions per mile	797	794	−3	−0.36%

True speed with the speedometer calibrated for 215/55R16
Indicated	Actual on 215/55R16	Actual on 235/40R18	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.2 mph	+0.2 mph
75 mph	75.0 mph	75.3 mph	+0.3 mph

Ride-height change: 235/40R18 raises the vehicle by 0.05 in (1.2 mm) at the axle (half the +2.3 mm diameter difference).

235/40R18 is 0.09 in taller than 215/55R16 and 20 mm wider, a 0.36% difference in overall diameter.

With the speedometer calibrated for 215/55R16, an indicated 60 mph is actually 60.2 mph on 235/40R18, and revolutions per mile change from 797 to 794.

235/40R18 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 235/40R18

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