285/40R21 vs 255/45R21

255/45R21 is 0.06 in (1.5 mm) taller than 285/40R21, a +0.20% diameter difference. With the speedometer set for 285/40R21, an indicated 60 mph is 60.1 mph on 255/45R21.

Is this a safe swap?

Diameter difference +0.20%: under 1%

By overall diameter alone, 255/45R21 is a near match for 285/40R21; 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.

285/40R21 vs 255/45R21 differences
Measurement	285/40R21	255/45R21	Difference	% diff
Overall diameter	29.98 in (761.4 mm)	30.04 in (762.9 mm)	+0.06 in (+1.5 mm)	+0.20%
Sidewall height	4.49 in (114.0 mm)	4.52 in (114.8 mm)	+0.03 in (+0.8 mm)	+0.66%
Section width	11.22 in (285.0 mm)	10.04 in (255.0 mm)	−1.18 in (−30.0 mm)	−10.53%
Circumference	94.17 in (2,392.0 mm)	94.36 in (2,396.7 mm)	+0.19 in (+4.7 mm)	+0.20%
Revolutions per mile	673	671	−1	−0.20%

True speed with the speedometer calibrated for 285/40R21
Indicated	Actual on 285/40R21	Actual on 255/45R21	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.1 mph	+0.1 mph
75 mph	75.0 mph	75.1 mph	+0.1 mph

Ride-height change: 255/45R21 raises the vehicle by 0.03 in (0.8 mm) at the axle (half the +1.5 mm diameter difference).

255/45R21 is 0.06 in taller than 285/40R21 and 30 mm narrower, a 0.20% difference in overall diameter.

With the speedometer calibrated for 285/40R21, an indicated 60 mph is actually 60.1 mph on 255/45R21, and revolutions per mile change from 673 to 671.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 285/40R21

Other comparisons with 255/45R21

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