235/75R17 vs 255/65R18

255/65R18 is 0.17 in (4.4 mm) taller than 235/75R17, a +0.56% diameter difference. With the speedometer set for 235/75R17, an indicated 60 mph is 60.3 mph on 255/65R18.

Is this a safe swap?

Diameter difference +0.56%: under 1%

By overall diameter alone, 255/65R18 is a near match for 235/75R17; 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.

235/75R17 vs 255/65R18 differences
Measurement	235/75R17	255/65R18	Difference	% diff
Overall diameter	30.88 in (784.3 mm)	31.05 in (788.7 mm)	+0.17 in (+4.4 mm)	+0.56%
Sidewall height	6.94 in (176.3 mm)	6.53 in (165.8 mm)	−0.41 in (−10.5 mm)	−5.96%
Section width	9.25 in (235.0 mm)	10.04 in (255.0 mm)	+0.79 in (+20.0 mm)	+8.51%
Circumference	97.01 in (2,464.0 mm)	97.55 in (2,477.8 mm)	+0.54 in (+13.8 mm)	+0.56%
Revolutions per mile	653	650	−4	−0.56%

True speed with the speedometer calibrated for 235/75R17
Indicated	Actual on 235/75R17	Actual on 255/65R18	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.3 mph	+0.3 mph
75 mph	75.0 mph	75.4 mph	+0.4 mph

Ride-height change: 255/65R18 raises the vehicle by 0.09 in (2.2 mm) at the axle (half the +4.4 mm diameter difference).

255/65R18 is 0.17 in taller than 235/75R17 and 20 mm wider, a 0.56% difference in overall diameter.

With the speedometer calibrated for 235/75R17, an indicated 60 mph is actually 60.3 mph on 255/65R18, and revolutions per mile change from 653 to 650.

255/65R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 235/75R17.

Size pages

Other comparisons with 235/75R17

Other comparisons with 255/65R18

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