235/55R20 vs 255/45R21

255/45R21 is 0.14 in (3.6 mm) shorter than 235/55R20, a −0.47% diameter difference. With the speedometer set for 235/55R20, an indicated 60 mph is 59.7 mph on 255/45R21.

Is this a safe swap?

Diameter difference −0.47%: under 1%

By overall diameter alone, 255/45R21 is a near match for 235/55R20; 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.

235/55R20 vs 255/45R21 differences
Measurement	235/55R20	255/45R21	Difference	% diff
Overall diameter	30.18 in (766.5 mm)	30.04 in (762.9 mm)	−0.14 in (−3.6 mm)	−0.47%
Sidewall height	5.09 in (129.3 mm)	4.52 in (114.8 mm)	−0.57 in (−14.5 mm)	−11.22%
Section width	9.25 in (235.0 mm)	10.04 in (255.0 mm)	+0.79 in (+20.0 mm)	+8.51%
Circumference	94.80 in (2,408.0 mm)	94.36 in (2,396.7 mm)	−0.45 in (−11.3 mm)	−0.47%
Revolutions per mile	668	671	+3	+0.47%

True speed with the speedometer calibrated for 235/55R20
Indicated	Actual on 235/55R20	Actual on 255/45R21	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.7 mph	−0.3 mph
75 mph	75.0 mph	74.6 mph	−0.4 mph

Ride-height change: 255/45R21 lowers the vehicle by 0.07 in (1.8 mm) at the axle (half the −3.6 mm diameter difference).

255/45R21 is 0.14 in shorter than 235/55R20 and 20 mm wider, a 0.47% difference in overall diameter.

With the speedometer calibrated for 235/55R20, an indicated 60 mph is actually 59.7 mph on 255/45R21, and revolutions per mile change from 668 to 671.

255/45R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 235/55R20.

Size pages

Other comparisons with 235/55R20

Other comparisons with 255/45R21

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