205/40R18 vs 235/35R18

235/35R18 is 0.02 in (0.5 mm) taller than 205/40R18, a +0.08% diameter difference. With the speedometer set for 205/40R18, an indicated 60 mph is 60.0 mph on 235/35R18.

Is this a safe swap?

Diameter difference +0.08%: under 1%

By overall diameter alone, 235/35R18 is a near match for 205/40R18; the speedometer changes by less than 1%. 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.

205/40R18 vs 235/35R18 differences
Measurement	205/40R18	235/35R18	Difference	% diff
Overall diameter	24.46 in (621.2 mm)	24.48 in (621.7 mm)	+0.02 in (+0.5 mm)	+0.08%
Sidewall height	3.23 in (82.0 mm)	3.24 in (82.3 mm)	+0.01 in (+0.3 mm)	+0.30%
Section width	8.07 in (205.0 mm)	9.25 in (235.0 mm)	+1.18 in (+30.0 mm)	+14.63%
Circumference	76.83 in (1,951.6 mm)	76.89 in (1,953.1 mm)	+0.06 in (+1.6 mm)	+0.08%
Revolutions per mile	825	824	−1	−0.08%

True speed with the speedometer calibrated for 205/40R18
Indicated	Actual on 205/40R18	Actual on 235/35R18	Difference
30 mph	30.0 mph	30.0 mph	0.0 mph
60 mph	60.0 mph	60.0 mph	0.0 mph
75 mph	75.0 mph	75.1 mph	+0.1 mph

Ride-height change: 235/35R18 raises the vehicle by 0.01 in (0.3 mm) at the axle (half the +0.5 mm diameter difference).

235/35R18 is 0.02 in taller than 205/40R18 and 30 mm wider, a 0.08% difference in overall diameter.

With the speedometer calibrated for 205/40R18, an indicated 60 mph is actually 60.0 mph on 235/35R18, and revolutions per mile change from 825 to 824.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 205/40R18

Other comparisons with 235/35R18

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