225/35R18 vs 205/40R18

205/40R18 is 0.26 in (6.5 mm) taller than 225/35R18, a +1.06% diameter difference. With the speedometer set for 225/35R18, an indicated 60 mph is 60.6 mph on 205/40R18.

Is this a safe swap?

Diameter difference +1.06%: within the ±3% guideline

By overall diameter alone, 205/40R18 falls inside the ±3% range commonly used as a guideline when replacing 225/35R18. 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.

225/35R18 vs 205/40R18 differences
Measurement	225/35R18	205/40R18	Difference	% diff
Overall diameter	24.20 in (614.7 mm)	24.46 in (621.2 mm)	+0.26 in (+6.5 mm)	+1.06%
Sidewall height	3.10 in (78.8 mm)	3.23 in (82.0 mm)	+0.13 in (+3.3 mm)	+4.13%
Section width	8.86 in (225.0 mm)	8.07 in (205.0 mm)	−0.79 in (−20.0 mm)	−8.89%
Circumference	76.03 in (1,931.1 mm)	76.83 in (1,951.6 mm)	+0.80 in (+20.4 mm)	+1.06%
Revolutions per mile	833	825	−9	−1.05%

True speed with the speedometer calibrated for 225/35R18
Indicated	Actual on 225/35R18	Actual on 205/40R18	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: 205/40R18 raises the vehicle by 0.13 in (3.3 mm) at the axle (half the +6.5 mm diameter difference).

205/40R18 is 0.26 in taller than 225/35R18 and 20 mm narrower, a 1.06% difference in overall diameter.

With the speedometer calibrated for 225/35R18, an indicated 60 mph is actually 60.6 mph on 205/40R18, and revolutions per mile change from 833 to 825.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 225/35R18

Other comparisons with 205/40R18

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