245/45R18 vs 285/30R20

285/30R20 is 0.05 in (1.3 mm) taller than 245/45R18, a +0.19% diameter difference. With the speedometer set for 245/45R18, an indicated 60 mph is 60.1 mph on 285/30R20.

Is this a safe swap?

Diameter difference +0.19%: under 1%

By overall diameter alone, 285/30R20 is a near match for 245/45R18; 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.

245/45R18 vs 285/30R20 differences
Measurement	245/45R18	285/30R20	Difference	% diff
Overall diameter	26.68 in (677.7 mm)	26.73 in (679.0 mm)	+0.05 in (+1.3 mm)	+0.19%
Sidewall height	4.34 in (110.3 mm)	3.37 in (85.5 mm)	−0.97 in (−24.7 mm)	−22.45%
Section width	9.65 in (245.0 mm)	11.22 in (285.0 mm)	+1.57 in (+40.0 mm)	+16.33%
Circumference	83.82 in (2,129.1 mm)	83.98 in (2,133.1 mm)	+0.16 in (+4.1 mm)	+0.19%
Revolutions per mile	756	754	−1	−0.19%

True speed with the speedometer calibrated for 245/45R18
Indicated	Actual on 245/45R18	Actual on 285/30R20	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.1 mph	+0.1 mph
75 mph	75.0 mph	75.1 mph	+0.1 mph

Ride-height change: 285/30R20 raises the vehicle by 0.03 in (0.6 mm) at the axle (half the +1.3 mm diameter difference).

285/30R20 is 0.05 in taller than 245/45R18 and 40 mm wider, a 0.19% difference in overall diameter.

With the speedometer calibrated for 245/45R18, an indicated 60 mph is actually 60.1 mph on 285/30R20, and revolutions per mile change from 756 to 754.

285/30R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 245/45R18.

Size pages

Other comparisons with 245/45R18

Other comparisons with 285/30R20

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