255/35R18 vs 225/30R20

225/30R20 is 0.29 in (7.3 mm) taller than 255/35R18, a +1.15% diameter difference. With the speedometer set for 255/35R18, an indicated 60 mph is 60.7 mph on 225/30R20.

Is this a safe swap?

Diameter difference +1.15%: within the ±3% guideline

By overall diameter alone, 225/30R20 falls inside the ±3% range commonly used as a guideline when replacing 255/35R18. 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.

255/35R18 vs 225/30R20 differences
Measurement	255/35R18	225/30R20	Difference	% diff
Overall diameter	25.03 in (635.7 mm)	25.31 in (643.0 mm)	+0.29 in (+7.3 mm)	+1.15%
Sidewall height	3.51 in (89.3 mm)	2.66 in (67.5 mm)	−0.86 in (−21.7 mm)	−24.37%
Section width	10.04 in (255.0 mm)	8.86 in (225.0 mm)	−1.18 in (−30.0 mm)	−11.76%
Circumference	78.63 in (1,997.1 mm)	79.53 in (2,020.0 mm)	+0.90 in (+22.9 mm)	+1.15%
Revolutions per mile	806	797	−9	−1.14%

True speed with the speedometer calibrated for 255/35R18
Indicated	Actual on 255/35R18	Actual on 225/30R20	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.7 mph	+0.7 mph
75 mph	75.0 mph	75.9 mph	+0.9 mph

Ride-height change: 225/30R20 raises the vehicle by 0.14 in (3.6 mm) at the axle (half the +7.3 mm diameter difference).

225/30R20 is 0.29 in taller than 255/35R18 and 30 mm narrower, a 1.15% difference in overall diameter.

With the speedometer calibrated for 255/35R18, an indicated 60 mph is actually 60.7 mph on 225/30R20, and revolutions per mile change from 806 to 797.

225/30R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 255/35R18.

Size pages

Other comparisons with 255/35R18

Other comparisons with 225/30R20

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