205/40R18 vs 225/40R18

225/40R18 is 0.63 in (16.0 mm) taller than 205/40R18, a +2.58% diameter difference. With the speedometer set for 205/40R18, an indicated 60 mph is 61.5 mph on 225/40R18.

Is this a safe swap?

Diameter difference +2.58%: within the ±3% guideline

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

205/40R18 vs 225/40R18 differences
Measurement	205/40R18	225/40R18	Difference	% diff
Overall diameter	24.46 in (621.2 mm)	25.09 in (637.2 mm)	+0.63 in (+16.0 mm)	+2.58%
Sidewall height	3.23 in (82.0 mm)	3.54 in (90.0 mm)	+0.31 in (+8.0 mm)	+9.76%
Section width	8.07 in (205.0 mm)	8.86 in (225.0 mm)	+0.79 in (+20.0 mm)	+9.76%
Circumference	76.83 in (1,951.6 mm)	78.81 in (2,001.8 mm)	+1.98 in (+50.3 mm)	+2.58%
Revolutions per mile	825	804	−21	−2.51%

True speed with the speedometer calibrated for 205/40R18
Indicated	Actual on 205/40R18	Actual on 225/40R18	Difference
30 mph	30.0 mph	30.8 mph	+0.8 mph
60 mph	60.0 mph	61.5 mph	+1.5 mph
75 mph	75.0 mph	76.9 mph	+1.9 mph

Ride-height change: 225/40R18 raises the vehicle by 0.31 in (8.0 mm) at the axle (half the +16.0 mm diameter difference).

225/40R18 is 0.63 in taller than 205/40R18 and 20 mm wider, a 2.58% difference in overall diameter.

With the speedometer calibrated for 205/40R18, an indicated 60 mph is actually 61.5 mph on 225/40R18, and revolutions per mile change from 825 to 804.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 205/40R18

Other comparisons with 225/40R18

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