235/65R18 vs 255/50R20

255/50R20 is 0.01 in (0.3 mm) taller than 235/65R18, a +0.04% diameter difference. With the speedometer set for 235/65R18, an indicated 60 mph is 60.0 mph on 255/50R20.

Is this a safe swap?

Diameter difference +0.04%: under 1%

By overall diameter alone, 255/50R20 is a near match for 235/65R18; 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.

235/65R18 vs 255/50R20 differences
Measurement	235/65R18	255/50R20	Difference	% diff
Overall diameter	30.03 in (762.7 mm)	30.04 in (763.0 mm)	+0.01 in (+0.3 mm)	+0.04%
Sidewall height	6.01 in (152.8 mm)	5.02 in (127.5 mm)	−0.99 in (−25.2 mm)	−16.53%
Section width	9.25 in (235.0 mm)	10.04 in (255.0 mm)	+0.79 in (+20.0 mm)	+8.51%
Circumference	94.33 in (2,396.1 mm)	94.37 in (2,397.0 mm)	+0.04 in (+0.9 mm)	+0.04%
Revolutions per mile	672	671	0	−0.04%

True speed with the speedometer calibrated for 235/65R18
Indicated	Actual on 235/65R18	Actual on 255/50R20
30 mph	30.0 mph	30.0 mph
60 mph	60.0 mph	60.0 mph
75 mph	75.0 mph	75.0 mph

Ride-height change: 255/50R20 raises the vehicle by 0.01 in (0.1 mm) at the axle (half the +0.3 mm diameter difference).

255/50R20 is 0.01 in taller than 235/65R18 and 20 mm wider, a 0.04% difference in overall diameter.

With the speedometer calibrated for 235/65R18, an indicated 60 mph is actually 60.0 mph on 255/50R20, and revolutions per mile change from 672 to 671.

255/50R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 235/65R18.

Size pages

Other comparisons with 235/65R18

Other comparisons with 255/50R20

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