285/70R18 vs 305/55R20

305/55R20 is 0.50 in (12.7 mm) shorter than 285/70R18, a −1.48% diameter difference. With the speedometer set for 285/70R18, an indicated 60 mph is 59.1 mph on 305/55R20.

Is this a safe swap?

Diameter difference −1.48%: within the ±3% guideline

By overall diameter alone, 305/55R20 falls inside the ±3% range commonly used as a guideline when replacing 285/70R18. 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.

285/70R18 vs 305/55R20 differences
Measurement	285/70R18	305/55R20	Difference	% diff
Overall diameter	33.71 in (856.2 mm)	33.21 in (843.5 mm)	−0.50 in (−12.7 mm)	−1.48%
Sidewall height	7.85 in (199.5 mm)	6.60 in (167.8 mm)	−1.25 in (−31.7 mm)	−15.91%
Section width	11.22 in (285.0 mm)	12.01 in (305.0 mm)	+0.79 in (+20.0 mm)	+7.02%
Circumference	105.90 in (2,689.8 mm)	104.33 in (2,649.9 mm)	−1.57 in (−39.9 mm)	−1.48%
Revolutions per mile	598	607	+9	+1.51%

True speed with the speedometer calibrated for 285/70R18
Indicated	Actual on 285/70R18	Actual on 305/55R20	Difference
30 mph	30.0 mph	29.6 mph	−0.4 mph
60 mph	60.0 mph	59.1 mph	−0.9 mph
75 mph	75.0 mph	73.9 mph	−1.1 mph

Ride-height change: 305/55R20 lowers the vehicle by 0.25 in (6.4 mm) at the axle (half the −12.7 mm diameter difference).

305/55R20 is 0.50 in shorter than 285/70R18 and 20 mm wider, a 1.48% difference in overall diameter.

With the speedometer calibrated for 285/70R18, an indicated 60 mph is actually 59.1 mph on 305/55R20, and revolutions per mile change from 598 to 607.

305/55R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 285/70R18.

Size pages

Other comparisons with 285/70R18

Other comparisons with 305/55R20

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