215/55R17 vs 235/45R18

235/45R18 is 0.02 in (0.4 mm) taller than 215/55R17, a +0.06% diameter difference. With the speedometer set for 215/55R17, an indicated 60 mph is 60.0 mph on 235/45R18.

Is this a safe swap?

Diameter difference +0.06%: under 1%

By overall diameter alone, 235/45R18 is a near match for 215/55R17; 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.

215/55R17 vs 235/45R18 differences
Measurement	215/55R17	235/45R18	Difference	% diff
Overall diameter	26.31 in (668.3 mm)	26.33 in (668.7 mm)	+0.02 in (+0.4 mm)	+0.06%
Sidewall height	4.66 in (118.3 mm)	4.16 in (105.8 mm)	−0.49 in (−12.5 mm)	−10.57%
Section width	8.46 in (215.0 mm)	9.25 in (235.0 mm)	+0.79 in (+20.0 mm)	+9.30%
Circumference	82.66 in (2,099.5 mm)	82.71 in (2,100.8 mm)	+0.05 in (+1.3 mm)	+0.06%
Revolutions per mile	767	766	0	−0.06%

True speed with the speedometer calibrated for 215/55R17
Indicated	Actual on 215/55R17	Actual on 235/45R18
30 mph	30.0 mph	30.0 mph
60 mph	60.0 mph	60.0 mph
75 mph	75.0 mph	75.0 mph

Ride-height change: 235/45R18 raises the vehicle by 0.01 in (0.2 mm) at the axle (half the +0.4 mm diameter difference).

235/45R18 is 0.02 in taller than 215/55R17 and 20 mm wider, a 0.06% difference in overall diameter.

With the speedometer calibrated for 215/55R17, an indicated 60 mph is actually 60.0 mph on 235/45R18, and revolutions per mile change from 767 to 766.

235/45R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 215/55R17.

Size pages

Other comparisons with 215/55R17

Other comparisons with 235/45R18

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