225/50R18 vs 285/40R18

285/40R18 is 0.12 in (3.0 mm) taller than 225/50R18, a +0.44% diameter difference. With the speedometer set for 225/50R18, an indicated 60 mph is 60.3 mph on 285/40R18.

Is this a safe swap?

Diameter difference +0.44%: under 1%

By overall diameter alone, 285/40R18 is a near match for 225/50R18; 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.

225/50R18 vs 285/40R18 differences
Measurement	225/50R18	285/40R18	Difference	% diff
Overall diameter	26.86 in (682.2 mm)	26.98 in (685.2 mm)	+0.12 in (+3.0 mm)	+0.44%
Sidewall height	4.43 in (112.5 mm)	4.49 in (114.0 mm)	+0.06 in (+1.5 mm)	+1.33%
Section width	8.86 in (225.0 mm)	11.22 in (285.0 mm)	+2.36 in (+60.0 mm)	+26.67%
Circumference	84.38 in (2,143.2 mm)	84.75 in (2,152.6 mm)	+0.37 in (+9.4 mm)	+0.44%
Revolutions per mile	751	748	−3	−0.44%

True speed with the speedometer calibrated for 225/50R18
Indicated	Actual on 225/50R18	Actual on 285/40R18	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.3 mph	+0.3 mph
75 mph	75.0 mph	75.3 mph	+0.3 mph

Ride-height change: 285/40R18 raises the vehicle by 0.06 in (1.5 mm) at the axle (half the +3.0 mm diameter difference).

285/40R18 is 0.12 in taller than 225/50R18 and 60 mm wider, a 0.44% difference in overall diameter.

With the speedometer calibrated for 225/50R18, an indicated 60 mph is actually 60.3 mph on 285/40R18, and revolutions per mile change from 751 to 748.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 225/50R18

Other comparisons with 285/40R18

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