225/50R18 vs 285/30R20

285/30R20 is 0.13 in (3.2 mm) shorter than 225/50R18, a −0.47% diameter difference. With the speedometer set for 225/50R18, an indicated 60 mph is 59.7 mph on 285/30R20.

Is this a safe swap?

Diameter difference −0.47%: under 1%

By overall diameter alone, 285/30R20 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 285/30R20 differences
Measurement	225/50R18	285/30R20	Difference	% diff
Overall diameter	26.86 in (682.2 mm)	26.73 in (679.0 mm)	−0.13 in (−3.2 mm)	−0.47%
Sidewall height	4.43 in (112.5 mm)	3.37 in (85.5 mm)	−1.06 in (−27.0 mm)	−24.00%
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)	83.98 in (2,133.1 mm)	−0.40 in (−10.1 mm)	−0.47%
Revolutions per mile	751	754	+4	+0.47%

True speed with the speedometer calibrated for 225/50R18
Indicated	Actual on 225/50R18	Actual on 285/30R20	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.7 mph	−0.3 mph
75 mph	75.0 mph	74.6 mph	−0.4 mph

Ride-height change: 285/30R20 lowers the vehicle by 0.06 in (1.6 mm) at the axle (half the −3.2 mm diameter difference).

285/30R20 is 0.13 in shorter than 225/50R18 and 60 mm wider, a 0.47% difference in overall diameter.

With the speedometer calibrated for 225/50R18, an indicated 60 mph is actually 59.7 mph on 285/30R20, and revolutions per mile change from 751 to 754.

285/30R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 225/50R18.

Size pages

Other comparisons with 225/50R18

Other comparisons with 285/30R20

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