TireSizesFinder

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.

To-scale drawing: 285/40R21 (29.98 in) vs 255/45R21 (30.04 in)285/40R2129.98 in × 11.22 in255/45R2130.04 in × 10.04 in
Side profile and tread width, drawn to the same scale.
285/40R21 vs 255/45R21 differences
Measurement285/40R21255/45R21Difference% diff
Overall diameter29.98 in (761.4 mm)30.04 in (762.9 mm)+0.06 in (+1.5 mm)+0.20%
Sidewall height4.49 in (114.0 mm)4.52 in (114.8 mm)+0.03 in (+0.8 mm)+0.66%
Section width11.22 in (285.0 mm)10.04 in (255.0 mm)−1.18 in (−30.0 mm)−10.53%
Circumference94.17 in (2,392.0 mm)94.36 in (2,396.7 mm)+0.19 in (+4.7 mm)+0.20%
Revolutions per mile673671−1−0.20%
True speed with the speedometer calibrated for 285/40R21
IndicatedActual on 285/40R21Actual on 255/45R21Difference
30 mph30.0 mph30.1 mph+0.1 mph
60 mph60.0 mph60.1 mph+0.1 mph
75 mph75.0 mph75.1 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 . How these numbers are calculated.