225/70R16 vs 235/55R18

235/55R18 is 0.22 in (5.7 mm) shorter than 225/70R16, a −0.79% diameter difference. With the speedometer set for 225/70R16, an indicated 60 mph is 59.5 mph on 235/55R18.

Is this a safe swap?

Diameter difference −0.79%: under 1%

By overall diameter alone, 235/55R18 is a near match for 225/70R16; 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.

225/70R16 vs 235/55R18 differences
Measurement	225/70R16	235/55R18	Difference	% diff
Overall diameter	28.40 in (721.4 mm)	28.18 in (715.7 mm)	−0.22 in (−5.7 mm)	−0.79%
Sidewall height	6.20 in (157.5 mm)	5.09 in (129.3 mm)	−1.11 in (−28.2 mm)	−17.94%
Section width	8.86 in (225.0 mm)	9.25 in (235.0 mm)	+0.39 in (+10.0 mm)	+4.44%
Circumference	89.23 in (2,266.3 mm)	88.52 in (2,248.4 mm)	−0.71 in (−17.9 mm)	−0.79%
Revolutions per mile	710	716	+6	+0.80%

True speed with the speedometer calibrated for 225/70R16
Indicated	Actual on 225/70R16	Actual on 235/55R18	Difference
30 mph	30.0 mph	29.8 mph	−0.2 mph
60 mph	60.0 mph	59.5 mph	−0.5 mph
75 mph	75.0 mph	74.4 mph	−0.6 mph

Ride-height change: 235/55R18 lowers the vehicle by 0.11 in (2.8 mm) at the axle (half the −5.7 mm diameter difference).

235/55R18 is 0.22 in shorter than 225/70R16 and 10 mm wider, a 0.79% difference in overall diameter.

With the speedometer calibrated for 225/70R16, an indicated 60 mph is actually 59.5 mph on 235/55R18, and revolutions per mile change from 710 to 716.

235/55R18 mounts on a 18-inch wheel, a plus-2 fitment from the 16-inch wheel of 225/70R16.

Size pages

Other comparisons with 225/70R16

Other comparisons with 235/55R18

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