215/35R18 vs 205/40R18

205/40R18 is 0.53 in (13.5 mm) taller than 215/35R18, a +2.22% diameter difference. With the speedometer set for 215/35R18, an indicated 60 mph is 61.3 mph on 205/40R18.

Is this a safe swap?

Diameter difference +2.22%: within the ±3% guideline

By overall diameter alone, 205/40R18 falls inside the ±3% range commonly used as a guideline when replacing 215/35R18. 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.

215/35R18 vs 205/40R18 differences
Measurement	215/35R18	205/40R18	Difference	% diff
Overall diameter	23.93 in (607.7 mm)	24.46 in (621.2 mm)	+0.53 in (+13.5 mm)	+2.22%
Sidewall height	2.96 in (75.3 mm)	3.23 in (82.0 mm)	+0.27 in (+6.8 mm)	+8.97%
Section width	8.46 in (215.0 mm)	8.07 in (205.0 mm)	−0.39 in (−10.0 mm)	−4.65%
Circumference	75.16 in (1,909.1 mm)	76.83 in (1,951.6 mm)	+1.67 in (+42.4 mm)	+2.22%
Revolutions per mile	843	825	−18	−2.17%

True speed with the speedometer calibrated for 215/35R18
Indicated	Actual on 215/35R18	Actual on 205/40R18	Difference
30 mph	30.0 mph	30.7 mph	+0.7 mph
60 mph	60.0 mph	61.3 mph	+1.3 mph
75 mph	75.0 mph	76.7 mph	+1.7 mph

Ride-height change: 205/40R18 raises the vehicle by 0.27 in (6.8 mm) at the axle (half the +13.5 mm diameter difference).

205/40R18 is 0.53 in taller than 215/35R18 and 10 mm narrower, a 2.22% difference in overall diameter.

With the speedometer calibrated for 215/35R18, an indicated 60 mph is actually 61.3 mph on 205/40R18, and revolutions per mile change from 843 to 825.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 215/35R18

Other comparisons with 205/40R18

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