215/40R18 vs 255/35R18

255/35R18 is 0.26 in (6.5 mm) taller than 215/40R18, a +1.03% diameter difference. With the speedometer set for 215/40R18, an indicated 60 mph is 60.6 mph on 255/35R18.

Is this a safe swap?

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

By overall diameter alone, 255/35R18 falls inside the ±3% range commonly used as a guideline when replacing 215/40R18. 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.

215/40R18 vs 255/35R18 differences
Measurement	215/40R18	255/35R18	Difference	% diff
Overall diameter	24.77 in (629.2 mm)	25.03 in (635.7 mm)	+0.26 in (+6.5 mm)	+1.03%
Sidewall height	3.39 in (86.0 mm)	3.51 in (89.3 mm)	+0.13 in (+3.3 mm)	+3.78%
Section width	8.46 in (215.0 mm)	10.04 in (255.0 mm)	+1.57 in (+40.0 mm)	+18.60%
Circumference	77.82 in (1,976.7 mm)	78.63 in (1,997.1 mm)	+0.80 in (+20.4 mm)	+1.03%
Revolutions per mile	814	806	−8	−1.02%

True speed with the speedometer calibrated for 215/40R18
Indicated	Actual on 215/40R18	Actual on 255/35R18	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: 255/35R18 raises the vehicle by 0.13 in (3.3 mm) at the axle (half the +6.5 mm diameter difference).

255/35R18 is 0.26 in taller than 215/40R18 and 40 mm wider, a 1.03% difference in overall diameter.

With the speedometer calibrated for 215/40R18, an indicated 60 mph is actually 60.6 mph on 255/35R18, and revolutions per mile change from 814 to 806.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 215/40R18

Other comparisons with 255/35R18

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