235/70R17 vs 255/60R18

255/60R18 is 0.09 in (2.4 mm) taller than 235/70R17, a +0.32% diameter difference. With the speedometer set for 235/70R17, an indicated 60 mph is 60.2 mph on 255/60R18.

Is this a safe swap?

Diameter difference +0.32%: under 1%

By overall diameter alone, 255/60R18 is a near match for 235/70R17; 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.

235/70R17 vs 255/60R18 differences
Measurement	235/70R17	255/60R18	Difference	% diff
Overall diameter	29.95 in (760.8 mm)	30.05 in (763.2 mm)	+0.09 in (+2.4 mm)	+0.32%
Sidewall height	6.48 in (164.5 mm)	6.02 in (153.0 mm)	−0.45 in (−11.5 mm)	−6.99%
Section width	9.25 in (235.0 mm)	10.04 in (255.0 mm)	+0.79 in (+20.0 mm)	+8.51%
Circumference	94.10 in (2,390.1 mm)	94.40 in (2,397.7 mm)	+0.30 in (+7.5 mm)	+0.32%
Revolutions per mile	673	671	−2	−0.31%

True speed with the speedometer calibrated for 235/70R17
Indicated	Actual on 235/70R17	Actual on 255/60R18	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: 255/60R18 raises the vehicle by 0.05 in (1.2 mm) at the axle (half the +2.4 mm diameter difference).

255/60R18 is 0.09 in taller than 235/70R17 and 20 mm wider, a 0.32% difference in overall diameter.

With the speedometer calibrated for 235/70R17, an indicated 60 mph is actually 60.2 mph on 255/60R18, and revolutions per mile change from 673 to 671.

255/60R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 235/70R17.

Size pages

Other comparisons with 235/70R17

Other comparisons with 255/60R18

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