225/50R18 vs 255/35R20

255/35R20 is 0.17 in (4.3 mm) taller than 225/50R18, a +0.63% diameter difference. With the speedometer set for 225/50R18, an indicated 60 mph is 60.4 mph on 255/35R20.

Is this a safe swap?

Diameter difference +0.63%: under 1%

By overall diameter alone, 255/35R20 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 255/35R20 differences
Measurement	225/50R18	255/35R20	Difference	% diff
Overall diameter	26.86 in (682.2 mm)	27.03 in (686.5 mm)	+0.17 in (+4.3 mm)	+0.63%
Sidewall height	4.43 in (112.5 mm)	3.51 in (89.3 mm)	−0.92 in (−23.2 mm)	−20.67%
Section width	8.86 in (225.0 mm)	10.04 in (255.0 mm)	+1.18 in (+30.0 mm)	+13.33%
Circumference	84.38 in (2,143.2 mm)	84.91 in (2,156.7 mm)	+0.53 in (+13.5 mm)	+0.63%
Revolutions per mile	751	746	−5	−0.63%

True speed with the speedometer calibrated for 225/50R18
Indicated	Actual on 225/50R18	Actual on 255/35R20	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.4 mph	+0.4 mph
75 mph	75.0 mph	75.5 mph	+0.5 mph

Ride-height change: 255/35R20 raises the vehicle by 0.08 in (2.1 mm) at the axle (half the +4.3 mm diameter difference).

255/35R20 is 0.17 in taller than 225/50R18 and 30 mm wider, a 0.63% difference in overall diameter.

With the speedometer calibrated for 225/50R18, an indicated 60 mph is actually 60.4 mph on 255/35R20, and revolutions per mile change from 751 to 746.

255/35R20 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 255/35R20

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