235/55R17 vs 215/55R18

215/55R18 is 0.13 in (3.4 mm) taller than 235/55R17, a +0.49% diameter difference. With the speedometer set for 235/55R17, an indicated 60 mph is 60.3 mph on 215/55R18.

Is this a safe swap?

Diameter difference +0.49%: under 1%

By overall diameter alone, 215/55R18 is a near match for 235/55R17; 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.

235/55R17 vs 215/55R18 differences
Measurement	235/55R17	215/55R18	Difference	% diff
Overall diameter	27.18 in (690.3 mm)	27.31 in (693.7 mm)	+0.13 in (+3.4 mm)	+0.49%
Sidewall height	5.09 in (129.3 mm)	4.66 in (118.3 mm)	−0.43 in (−11.0 mm)	−8.51%
Section width	9.25 in (235.0 mm)	8.46 in (215.0 mm)	−0.79 in (−20.0 mm)	−8.51%
Circumference	85.38 in (2,168.6 mm)	85.80 in (2,179.3 mm)	+0.42 in (+10.7 mm)	+0.49%
Revolutions per mile	742	738	−4	−0.49%

True speed with the speedometer calibrated for 235/55R17
Indicated	Actual on 235/55R17	Actual on 215/55R18	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.3 mph	+0.3 mph
75 mph	75.0 mph	75.4 mph	+0.4 mph

Ride-height change: 215/55R18 raises the vehicle by 0.07 in (1.7 mm) at the axle (half the +3.4 mm diameter difference).

215/55R18 is 0.13 in taller than 235/55R17 and 20 mm narrower, a 0.49% difference in overall diameter.

With the speedometer calibrated for 235/55R17, an indicated 60 mph is actually 60.3 mph on 215/55R18, and revolutions per mile change from 742 to 738.

215/55R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 235/55R17.

Size pages

Other comparisons with 235/55R17

Other comparisons with 215/55R18

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