235/45R20 vs 255/45R20

255/45R20 is 0.71 in (18.0 mm) taller than 235/45R20, a +2.50% diameter difference. With the speedometer set for 235/45R20, an indicated 60 mph is 61.5 mph on 255/45R20.

Is this a safe swap?

Diameter difference +2.50%: within the ±3% guideline

By overall diameter alone, 255/45R20 falls inside the ±3% range commonly used as a guideline when replacing 235/45R20. 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.

235/45R20 vs 255/45R20 differences
Measurement	235/45R20	255/45R20	Difference	% diff
Overall diameter	28.33 in (719.5 mm)	29.04 in (737.5 mm)	+0.71 in (+18.0 mm)	+2.50%
Sidewall height	4.16 in (105.8 mm)	4.52 in (114.8 mm)	+0.35 in (+9.0 mm)	+8.51%
Section width	9.25 in (235.0 mm)	10.04 in (255.0 mm)	+0.79 in (+20.0 mm)	+8.51%
Circumference	88.99 in (2,260.4 mm)	91.22 in (2,316.9 mm)	+2.23 in (+56.5 mm)	+2.50%
Revolutions per mile	712	695	−17	−2.44%

True speed with the speedometer calibrated for 235/45R20
Indicated	Actual on 235/45R20	Actual on 255/45R20	Difference
30 mph	30.0 mph	30.8 mph	+0.8 mph
60 mph	60.0 mph	61.5 mph	+1.5 mph
75 mph	75.0 mph	76.9 mph	+1.9 mph

Ride-height change: 255/45R20 raises the vehicle by 0.35 in (9.0 mm) at the axle (half the +18.0 mm diameter difference).

255/45R20 is 0.71 in taller than 235/45R20 and 20 mm wider, a 2.50% difference in overall diameter.

With the speedometer calibrated for 235/45R20, an indicated 60 mph is actually 61.5 mph on 255/45R20, and revolutions per mile change from 712 to 695.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 235/45R20

Other comparisons with 255/45R20

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