235/35R18 vs 215/40R18

215/40R18 is 0.30 in (7.5 mm) taller than 235/35R18, a +1.21% diameter difference. With the speedometer set for 235/35R18, an indicated 60 mph is 60.7 mph on 215/40R18.

Is this a safe swap?

Diameter difference +1.21%: within the ±3% guideline

By overall diameter alone, 215/40R18 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 215/40R18 differences
Measurement	235/35R18	215/40R18	Difference	% diff
Overall diameter	24.48 in (621.7 mm)	24.77 in (629.2 mm)	+0.30 in (+7.5 mm)	+1.21%
Sidewall height	3.24 in (82.3 mm)	3.39 in (86.0 mm)	+0.15 in (+3.8 mm)	+4.56%
Section width	9.25 in (235.0 mm)	8.46 in (215.0 mm)	−0.79 in (−20.0 mm)	−8.51%
Circumference	76.89 in (1,953.1 mm)	77.82 in (1,976.7 mm)	+0.93 in (+23.6 mm)	+1.21%
Revolutions per mile	824	814	−10	−1.19%

True speed with the speedometer calibrated for 235/35R18
Indicated	Actual on 235/35R18	Actual on 215/40R18	Difference
30 mph	30.0 mph	30.4 mph	+0.4 mph
60 mph	60.0 mph	60.7 mph	+0.7 mph
75 mph	75.0 mph	75.9 mph	+0.9 mph

Ride-height change: 215/40R18 raises the vehicle by 0.15 in (3.8 mm) at the axle (half the +7.5 mm diameter difference).

215/40R18 is 0.30 in taller than 235/35R18 and 20 mm narrower, a 1.21% difference in overall diameter.

With the speedometer calibrated for 235/35R18, an indicated 60 mph is actually 60.7 mph on 215/40R18, and revolutions per mile change from 824 to 814.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 235/35R18

Other comparisons with 215/40R18

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