255/50R20 vs 255/45R21

255/45R21 is 0.00 in (0.1 mm) shorter than 255/50R20, a −0.01% diameter difference. With the speedometer set for 255/50R20, an indicated 60 mph is 60.0 mph on 255/45R21.

Is this a safe swap?

Diameter difference −0.01%: under 1%

By overall diameter alone, 255/45R21 is a near match for 255/50R20; 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.

255/50R20 vs 255/45R21 differences
Measurement	255/50R20	255/45R21	Difference	% diff
Overall diameter	30.04 in (763.0 mm)	30.04 in (762.9 mm)	0.00 in (−0.1 mm)	−0.01%
Sidewall height	5.02 in (127.5 mm)	4.52 in (114.8 mm)	−0.50 in (−12.7 mm)	−10.00%
Section width	10.04 in (255.0 mm)	10.04 in (255.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	94.37 in (2,397.0 mm)	94.36 in (2,396.7 mm)	−0.01 in (−0.3 mm)	−0.01%
Revolutions per mile	671	671	0	+0.01%

True speed with the speedometer calibrated for 255/50R20
Indicated	Actual on 255/50R20	Actual on 255/45R21
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: 255/45R21 lowers the vehicle by 0.00 in (0.1 mm) at the axle (half the −0.1 mm diameter difference).

255/45R21 is the same height as 255/50R20 and the same width, a 0.01% difference in overall diameter.

With the speedometer calibrated for 255/50R20, an indicated 60 mph is actually 60.0 mph on 255/45R21, and revolutions per mile change from 671 to 671.

255/45R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 255/50R20.

Size pages

Other comparisons with 255/50R20

Other comparisons with 255/45R21

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