235/35R18 vs 255/35R18

255/35R18 is 0.55 in (14.0 mm) taller than 235/35R18, a +2.25% diameter difference. With the speedometer set for 235/35R18, an indicated 60 mph is 61.4 mph on 255/35R18.

Is this a safe swap?

Diameter difference +2.25%: within the ±3% guideline

By overall diameter alone, 255/35R18 falls inside the ±3% range commonly used as a guideline when replacing 235/35R18. 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.

235/35R18 vs 255/35R18 differences
Measurement	235/35R18	255/35R18	Difference	% diff
Overall diameter	24.48 in (621.7 mm)	25.03 in (635.7 mm)	+0.55 in (+14.0 mm)	+2.25%
Sidewall height	3.24 in (82.3 mm)	3.51 in (89.3 mm)	+0.28 in (+7.0 mm)	+8.51%
Section width	9.25 in (235.0 mm)	10.04 in (255.0 mm)	+0.79 in (+20.0 mm)	+8.51%
Circumference	76.89 in (1,953.1 mm)	78.63 in (1,997.1 mm)	+1.73 in (+44.0 mm)	+2.25%
Revolutions per mile	824	806	−18	−2.20%

True speed with the speedometer calibrated for 235/35R18
Indicated	Actual on 235/35R18	Actual on 255/35R18	Difference
30 mph	30.0 mph	30.7 mph	+0.7 mph
60 mph	60.0 mph	61.4 mph	+1.4 mph
75 mph	75.0 mph	76.7 mph	+1.7 mph

Ride-height change: 255/35R18 raises the vehicle by 0.28 in (7.0 mm) at the axle (half the +14.0 mm diameter difference).

255/35R18 is 0.55 in taller than 235/35R18 and 20 mm wider, a 2.25% difference in overall diameter.

With the speedometer calibrated for 235/35R18, an indicated 60 mph is actually 61.4 mph on 255/35R18, and revolutions per mile change from 824 to 806.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 235/35R18

Other comparisons with 255/35R18

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