235/40R18 vs 255/40R18

255/40R18 is 0.63 in (16.0 mm) taller than 235/40R18, a +2.48% diameter difference. With the speedometer set for 235/40R18, an indicated 60 mph is 61.5 mph on 255/40R18.

Is this a safe swap?

Diameter difference +2.48%: within the ±3% guideline

By overall diameter alone, 255/40R18 falls inside the ±3% range commonly used as a guideline when replacing 235/40R18. 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.

235/40R18 vs 255/40R18 differences
Measurement	235/40R18	255/40R18	Difference	% diff
Overall diameter	25.40 in (645.2 mm)	26.03 in (661.2 mm)	+0.63 in (+16.0 mm)	+2.48%
Sidewall height	3.70 in (94.0 mm)	4.02 in (102.0 mm)	+0.31 in (+8.0 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	79.80 in (2,027.0 mm)	81.78 in (2,077.2 mm)	+1.98 in (+50.3 mm)	+2.48%
Revolutions per mile	794	775	−19	−2.42%

True speed with the speedometer calibrated for 235/40R18
Indicated	Actual on 235/40R18	Actual on 255/40R18	Difference
30 mph	30.0 mph	30.7 mph	+0.7 mph
60 mph	60.0 mph	61.5 mph	+1.5 mph
75 mph	75.0 mph	76.9 mph	+1.9 mph

Ride-height change: 255/40R18 raises the vehicle by 0.31 in (8.0 mm) at the axle (half the +16.0 mm diameter difference).

255/40R18 is 0.63 in taller than 235/40R18 and 20 mm wider, a 2.48% difference in overall diameter.

With the speedometer calibrated for 235/40R18, an indicated 60 mph is actually 61.5 mph on 255/40R18, and revolutions per mile change from 794 to 775.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 235/40R18

Other comparisons with 255/40R18

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