215/65R17 vs 235/55R18

235/55R18 is 0.17 in (4.4 mm) taller than 215/65R17, a +0.62% diameter difference. With the speedometer set for 215/65R17, an indicated 60 mph is 60.4 mph on 235/55R18.

Is this a safe swap?

Diameter difference +0.62%: under 1%

By overall diameter alone, 235/55R18 is a near match for 215/65R17; 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.

215/65R17 vs 235/55R18 differences
Measurement	215/65R17	235/55R18	Difference	% diff
Overall diameter	28.00 in (711.3 mm)	28.18 in (715.7 mm)	+0.17 in (+4.4 mm)	+0.62%
Sidewall height	5.50 in (139.8 mm)	5.09 in (129.3 mm)	−0.41 in (−10.5 mm)	−7.51%
Section width	8.46 in (215.0 mm)	9.25 in (235.0 mm)	+0.79 in (+20.0 mm)	+9.30%
Circumference	87.98 in (2,234.6 mm)	88.52 in (2,248.4 mm)	+0.54 in (+13.8 mm)	+0.62%
Revolutions per mile	720	716	−4	−0.61%

True speed with the speedometer calibrated for 215/65R17
Indicated	Actual on 215/65R17	Actual on 235/55R18	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.4 mph	+0.4 mph
75 mph	75.0 mph	75.5 mph	+0.5 mph

Ride-height change: 235/55R18 raises the vehicle by 0.09 in (2.2 mm) at the axle (half the +4.4 mm diameter difference).

235/55R18 is 0.17 in taller than 215/65R17 and 20 mm wider, a 0.62% difference in overall diameter.

With the speedometer calibrated for 215/65R17, an indicated 60 mph is actually 60.4 mph on 235/55R18, and revolutions per mile change from 720 to 716.

235/55R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 215/65R17.

Size pages

Other comparisons with 215/65R17

Other comparisons with 235/55R18

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