235/70R16 vs 255/55R18

255/55R18 is 0.09 in (2.3 mm) taller than 235/70R16, a +0.31% diameter difference. With the speedometer set for 235/70R16, an indicated 60 mph is 60.2 mph on 255/55R18.

Is this a safe swap?

Diameter difference +0.31%: under 1%

By overall diameter alone, 255/55R18 is a near match for 235/70R16; 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.

235/70R16 vs 255/55R18 differences
Measurement	235/70R16	255/55R18	Difference	% diff
Overall diameter	28.95 in (735.4 mm)	29.04 in (737.7 mm)	+0.09 in (+2.3 mm)	+0.31%
Sidewall height	6.48 in (164.5 mm)	5.52 in (140.3 mm)	−0.95 in (−24.2 mm)	−14.74%
Section width	9.25 in (235.0 mm)	10.04 in (255.0 mm)	+0.79 in (+20.0 mm)	+8.51%
Circumference	90.96 in (2,310.3 mm)	91.24 in (2,317.6 mm)	+0.28 in (+7.2 mm)	+0.31%
Revolutions per mile	697	694	−2	−0.31%

True speed with the speedometer calibrated for 235/70R16
Indicated	Actual on 235/70R16	Actual on 255/55R18	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	75.2 mph	+0.2 mph

Ride-height change: 255/55R18 raises the vehicle by 0.05 in (1.2 mm) at the axle (half the +2.3 mm diameter difference).

255/55R18 is 0.09 in taller than 235/70R16 and 20 mm wider, a 0.31% difference in overall diameter.

With the speedometer calibrated for 235/70R16, an indicated 60 mph is actually 60.2 mph on 255/55R18, and revolutions per mile change from 697 to 694.

255/55R18 mounts on a 18-inch wheel, a plus-2 fitment from the 16-inch wheel of 235/70R16.

Size pages

Other comparisons with 235/70R16

Other comparisons with 255/55R18

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