235/85R16 vs 255/70R18

255/70R18 is 0.33 in (8.3 mm) taller than 235/85R16, a +1.03% diameter difference. With the speedometer set for 235/85R16, an indicated 60 mph is 60.6 mph on 255/70R18.

Is this a safe swap?

Diameter difference +1.03%: within the ±3% guideline

By overall diameter alone, 255/70R18 falls inside the ±3% range commonly used as a guideline when replacing 235/85R16. 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.

235/85R16 vs 255/70R18 differences
Measurement	235/85R16	255/70R18	Difference	% diff
Overall diameter	31.73 in (805.9 mm)	32.06 in (814.2 mm)	+0.33 in (+8.3 mm)	+1.03%
Sidewall height	7.86 in (199.8 mm)	7.03 in (178.5 mm)	−0.84 in (−21.2 mm)	−10.64%
Section width	9.25 in (235.0 mm)	10.04 in (255.0 mm)	+0.79 in (+20.0 mm)	+8.51%
Circumference	99.68 in (2,531.8 mm)	100.70 in (2,557.9 mm)	+1.03 in (+26.1 mm)	+1.03%
Revolutions per mile	636	629	−6	−1.02%

True speed with the speedometer calibrated for 235/85R16
Indicated	Actual on 235/85R16	Actual on 255/70R18	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.6 mph	+0.6 mph
75 mph	75.0 mph	75.8 mph	+0.8 mph

Ride-height change: 255/70R18 raises the vehicle by 0.16 in (4.2 mm) at the axle (half the +8.3 mm diameter difference).

255/70R18 is 0.33 in taller than 235/85R16 and 20 mm wider, a 1.03% difference in overall diameter.

With the speedometer calibrated for 235/85R16, an indicated 60 mph is actually 60.6 mph on 255/70R18, and revolutions per mile change from 636 to 629.

255/70R18 mounts on a 18-inch wheel, a plus-2 fitment from the 16-inch wheel of 235/85R16.

Size pages

Other comparisons with 235/85R16

Other comparisons with 255/70R18

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